Add new API to check whether a Bitmap was modified.

Bitmap.getGenerationId() can be used by caches to find out if a Bitmap has been
modified. This simply exposes an existing Skia API.

This change also adds a small test app for Canvas hardware acceleration. The new
Bitmap API is required to implement a texture cache.

Change-Id: I8547b146cd14c8afe1a2327fcd6d71b1b1cb68fc