The drawables cache strikes again

Bug #7117785

Draawables created from the ConstantState cache found in Resources must be
mutated before they can be safely modified by apps. Failure to do so results
in all drawables sharing the same constant state to be affected by the
modification.

In the case of the bugreport above, the status bar code plays tricks with
a background drawable and modifies its color to implement a fade in/out
effect. This drawable comes from a cached resource (color 0x0) and the
modifications made by the status bar apply to other clients of this drawable,
most notably the recents panel.

This change fixes several things:
- Simplifies colors caching by removing the assetCookie from the key. This
should result in better reuse of cached drawables
- Makes View.setBackgroundColor() honor the mutate() contract
- Ensure StateListDrawable properly mutates its children before modifying
them
- Optimize Bitmap/ColorDrawable to mark them mutated when they are not
created from an existing ConstantSate. The same optimization should be
applied to other drawables in the future

Change-Id: I54adb5d5b914c7d8930bf9b46f7e3f9dcbf4bcab