When detached from window, the last known RequestKey is saved inside of the...

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