Major refactor:
Instead of caching Drawables, cache the actual bitmaps themselves.
This was necessary, because Drawables are mutated via their corresponding ImageViews
via setBounds etc.
For example, if an ImageView scales a drawable down, this has the effect of scaling
the drawawble down for all other ImageViews using the same drawable.
By caching bitmaps, and refcounting using the finalizer, unique Bitmap/ZombieDrawable
instances can be used. Under the hood, the Bitmap is still cached and used
by multiple Drawables.

There were also breaking API changes (regarding loaders and callbacks) that could not
be avoided without gimping the API.