When detached from window, the last known RequestKey is saved inside of the BasicBitmapDrawable instead of referenced separately in the BitmapDrawableImageView. This fixes a bug where you could bind() a stale RequestKey to the drawable, after having called drawable.unbind() before it is attached back to the window.

Change-Id: Idba62fea647871d5c1c495b2c89e10779289a53b