Encapsulate Canvas.mNativeCanvas

Currently, the native canvas is accessed/manipulated from several
unrelated classes.

In order to facilitate SaveFlags emulation, this CL encapsulates
the field and refactors its external users.

Two main changes:

* new getNativeCanvas() getter for use in Java-level clients.
* JNI canvas swappers (GraphicsBuffers, Surface, TextureView &
  AssetAtlasService) are refactored based on the exising/equivalent
  safeCanvasSwap() Canvas method.

Change-Id: I966bd4898f0838fb3699e226d3d3d51e0224ea97