设置窗口透明度，Activity窗口背景变灰

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   public static void setWindowAlpha(Activity activity, float alpha) {
        if (alpha < 0 || alpha > 1) return;
        WindowManager.LayoutParams windowLP = activity.getWindow().getAttributes();
        windowLP.alpha = alpha;
        if (alpha == 1) {
            activity.getWindow().clearFlags(WindowManager.LayoutParams.FLAG_DIM_BEHIND);//不移除该Flag的话,在有视频的页面上的视频会出现黑屏的bug
        } else {
            activity.getWindow().addFlags(WindowManager.LayoutParams.FLAG_DIM_BEHIND);//此行代码主要是解决在华为手机上半透明效果无效的bug
        }
        activity.getWindow().setAttributes(windowLP);
    }