在自己的项目中实现的，没有做什么封装和通用性的使用，可用就行了。实现的原理就是采用一个通用的布局，利用ActivityLifecycle监听Activity的生命周期动态的添加和删除浮动按钮。

FloatButton

参考了一位大神的代码，忘了源码地址了。

package com.newbox.jnitest;

import android.app.Activity;
import android.content.Context;
import android.util.AttributeSet;
import android.view.LayoutInflater;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.widget.FrameLayout;
import android.widget.RelativeLayout;

public abstract class FloatButton extends RelativeLayout implements View.OnClickListener {
    private int parentHeight;//悬浮的父布局高度
    private int parentWidth;
    private FrameLayout flRoot;

    public FloatButton(Context context) {
        this(context, null, 0);
    }

    public FloatButton(Context context, AttributeSet attrs) {
        this(context, attrs, 0);

    }

    public FloatButton(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        View view = LayoutInflater.from(context).inflate(R.layout.float_button, this);
        renderView(view);
    }


    public void show(Activity activity, FrameLayout.LayoutParams layoutParams) {

        flRoot = (FrameLayout) activity.getWindow().getDecorView();

        this.setId(R.id.float_button);
        flRoot.addView(this, layoutParams);
    }

    public void dismiss() {
        if (flRoot != null) {
            flRoot.removeView(this);
        }
    }


    public void renderView(View view) {
        view.setOnClickListener(this);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        measureChildren(widthMeasureSpec, heightMeasureSpec);
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

    @Override
    protected void onLayout(boolean b, int i, int i1, int i2, int i3) {
        View view = getChildAt(0);
        view.layout(0, 0, view.getMeasuredWidth(), view.getMeasuredHeight());
    }


    private boolean isDrag;
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int rawX = (int) event.getRawX();
        int rawY = (int) event.getRawY();
        switch (event.getAction() & MotionEvent.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN:
                setPressed(true);//默认是点击事件
                isDrag = false;//默认是非拖动而是点击事件
                getParent().requestDisallowInterceptTouchEvent(true);//父布局不要拦截子布局的监听
                BaseApplication.lastX = rawX;
                BaseApplication.lastY = rawY;
                ViewGroup parent;
                if (getParent() != null) {
                    parent = (ViewGroup) getParent();
                    parentHeight = parent.getHeight();
                    parentWidth = parent.getWidth();
                }
                break;
            case MotionEvent.ACTION_MOVE:
                isDrag = (parentHeight > 0 && parentWidth > 0);//只有父布局存在你才可以拖动
                if (!isDrag) break;

                int dx = rawX - BaseApplication.lastX;
                int dy = rawY - BaseApplication.lastY;
                //这里修复一些华为手机无法触发点击事件
                int distance = (int) Math.sqrt(dx * dx + dy * dy);
                isDrag = distance > 0;//只有位移大于0说明拖动了
                if (!isDrag) break;

                float x = getX() + dx;
                float y = getY() + dy;
                //检测是否到达边缘 左上右下
                x = x < 0 ? 0 : x > parentWidth - getWidth() ? parentWidth - getWidth() : x;
                y = y < 0 ? 0 : y > parentHeight - getHeight() ? parentHeight - getHeight() : y;
                setX(x);
                setY(y);
                BaseApplication.lastX = rawX;
                BaseApplication.lastY = rawY;
                break;
            case MotionEvent.ACTION_UP:
                //如果是拖动状态下即非点击按压事件
                setPressed(!isDrag);
                break;
        }

        //如果不是拖拽，那么就不消费这个事件，以免影响点击事件的处理
        //拖拽事件要自己消费
        return isDrag || super.onTouchEvent(event);
    }

}

BaseApplication里面主要代码

 @Override
    public void onCreate() {
        super.onCreate();
        initLifeCycle();
    }

    public static int lastX;
    public static int lastY;
    private WeakReference<FloatButton> wrf;
    private FrameLayout.LayoutParams layoutParams;

    private void initLifeCycle() {
        DisplayMetrics displayMetrics = this.getResources().getDisplayMetrics();
        lastX = displayMetrics.widthPixels - 80;
        lastY = displayMetrics.heightPixels * 2 / 3;
        this.registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(final Activity activity, Bundle bundle) {


            }

            @Override
            public void onActivityStarted(Activity activity) {

            }

            @Override
            public void onActivityResumed(final Activity activity) {
                FloatButton button;
                if (wrf == null || wrf.get() == null) {
                    button = new FloatButton(getApplicationContext()) {
                        @Override
                        public void onClick(View v) {
                            activity.startActivity(new Intent(activity, SecondActivity.class));
//                            Intent intent = new Intent();
//                            intent.setAction("android.intent.action.VIEW");
//                            Uri content_url = Uri.parse("http://www.cnblogs.com");
//                            intent.setData(content_url);
//                            activity.startActivity(intent);
                        }
                    };
                    wrf = new WeakReference<>(button);
                } else {
                    button = wrf.get();
                }
                if (layoutParams == null) {

                    layoutParams = new FrameLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,
                            ViewGroup.LayoutParams.WRAP_CONTENT);
                    layoutParams.topMargin = BaseApplication.lastY;
                    layoutParams.leftMargin = BaseApplication.lastX;
                    layoutParams.width = 80;
                    layoutParams.height = 80;
                }

                button.show(activity, layoutParams);
            }

            @Override
            public void onActivityPaused(Activity activity) {
                FloatButton button = activity.findViewById(R.id.float_button);
                if (button != null) {
                    button.dismiss();
                }

            }

            @Override
            public void onActivityStopped(Activity activity) {

            }

            @Override
            public void onActivitySaveInstanceState(Activity activity, Bundle outState) {

            }

            @Override
            public void onActivityDestroyed(Activity activity) {

            }
        });
    }

基本实现了效果，如果需要在某些界面不显示的话，在onActivityResumed、onActivityPaused做过滤处理。
目前已知的Bug是，有小概率的在某些界面回到了默认的位置。