@Override
    protected final void onPostExecute(Bitmap bitmap) {
        onBitmapFileReady(bitmap);
    }
The {@link AsyncTask} handles:
 1. Inflate a fallback view when the requested image is not ready.
 2. Load the image from given uri generated from {@link BrowserServiceFileProvider}.
 3. Update the UI when image is ready.
To use this class:
 1. Override handlePreLoadingFallback() to inflate a fallback UI when image is not ready.
 2. Override onBitmapFileReady(Bitmap) to update the UI.
customtabs/src/android/support/customtabs/browseractions/BrowserServiceImageReadTask.java