Cache drawable constant states instead of drawables

Bug #5678369

Caching drawables directly in a static map was responsible for runtime
restarts. If two different UI threads requested the same drawable, the
first thread to issue the request would be given a drawable callback
belonging to the second thread. This would cause an exception in
ViewRootImpl on invalidate.

The solution is to store the drawable states and to create a new
drawable instance every time a drawable is requested from the
cache. This is similar to how preloaded resources are handled.

Change-Id: I47e24e2a168cf67a3589185c6cd77b70f9a1c7cf