Improve ImageView drawable re-use

Bug 22403868

Initial attempt only helped if setImageBitmap() was the only
thing called but during new-loading content it's common for a
placeholder to be set via setImageDrawable.

Tweak ImageView slightly to just have a BitmapDrawable that it
lazy-creates but will hold on to for any subsequent calls
to setImageBitmap

Change-Id: I7380521c7b363d458e4cda041f1f8b2b1fb3a93a