latch transparent region hint only when we get a new frame

since the transparent region hint really depends on the
content of the window containing the SurfaceView
(it's calculated by the view hierarchy based on
overlapping views), it makes sense to latch it only when
the content of the window (the app) changes.

This should help fixing drawing artifacts when changing the
layout of a window containing a SurfaceView.

Bug: 8511430
Change-Id: Ic3aa668495293cb4d82a2cd7dcf3b6a337287678