Cache SkImage in SurfaceTexture only if buffer content is same

SkImage is immutable and changing GraphicBuffer content after
the object is created is not allowed. We still can cache
the image when BufferQueue is empty and showing unchanged
buffer.

Test: Ran UiRendering CTS, camera app
Change-Id: I9edefbda534e5d15b41bd7ff804fb29b1f72cc75