private void showSplashScreen() {
        mSplashImage = Utils.convertDrawableToBitmap(mActivity, mDrawableId);
        if (mSplashImage == null) {
            Log.w(TAG, "Failed to retrieve splash image from provided drawable id");
            return;
        }
        ImageView view = new ImageView(mActivity);
        view.setLayoutParams(new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
        view.setImageBitmap(mSplashImage);
        view.setBackgroundColor(mBackgroundColor);

        view.setScaleType(mScaleType);
        if (mScaleType == ImageView.ScaleType.MATRIX) {
            view.setImageMatrix(mTransformationMatrix);
        }

        mActivity.setContentView(view);
    }
Splash screen is shown both before the Trusted Web Activity is launched - in this activity,
and for some time after that - in browser, on top of web page being loaded.
This method shows the splash screen in the LauncherActivity.
customtabs/src/android/support/customtabs/trusted/splashscreens/PwaWrapperSplashScreenStrategy.java