Don't preload textures for AssetAtlas

Bug: 18317479

RenderNode::prepareSubTree calls prefetchAndMarkInUse
on every bitmapResoruce in the DisplayList. However,
this resulted in textures being uploaded for bitmaps
that would be drawn from the AssetAtlas instead.

To fix this we teach TextureCache about the AssetAtlas
so that calls to TextureCache return the Texture from
AssetAtlas if it exists. Thus usage of AssetAtlas
is now purely to allow for further optimizations via
draw merging instead of a requirement to get
any benefit at all.

Change-Id: I65282fa05bac46f4e93822b3467ffa0261ccf200