Canvas JNI cleanup.

As pointed out in a previous patch review, some native Canvas methods do not
receive a native canvas handle and instead have to use GraphicsJNI to query
the object mNativeCanvasWrapper field.

To avoid the extra JNI call, this change adds the missing Java wrapper
methods and ensures that all native calls are passed efficient native
handles.

Also, some native methods are consolidated: save, saveAll -> save,
clipRect* -> clipRect.

Change-Id: I2eae4a633f6a463d9fbae2ca632661acc8b5cfc3