Track the atlas' generation ID

Bug #9589379

If the atlas is terminated/reinitialized and a view does not invalidate
in between it might end up using a stale AssetAtlas::Entry. This change
is similar to how 9patch meshes are cached in DrawPatchOp: we simply
track the generation ID of the cache to make sure we always use the
latest data.

Change-Id: Ib5abb3769d2ce0eabe9adc04e320ca27c422019e