SurfaceView: Expose hook for subclass to position child surfaces.

To position child surfaces of a SurfaceView in sync with the main view root
we need to expose the frame number and RT callback. This is of course not API
but we plan to use it with app controlled transitions, for manipulating window
surfaces within a view hierarchy.

Bug: 70341013
Test: Builds
Change-Id: Ibb3645461fac36bda8ab163867fcb2087cadee37