Cache VectorDrawables in an atlas

Optimize VectorDrawables for Skia pipeline: draw small VectorDrawables
in a GPU atlas instead of seprate offscreen buffers.
This implementation is using CacheManger and allows for the atlas to
be released if there is a memory pressure.

Test: A new unit test for VectorDrawableAtlas is passing. Systrace shows
0.5ms faster DrawFrame for fling in Settings app main screen.
Change-Id: Ide3884eefae777e1547f1dfdb67b807185839fb4