View: Expose gatherTransparentRegion

Recently exposed ViewRoot API requires requestTransparentRegion for
many of the use cases we want to support. requestTransparentRegion is
already public API, however the documentation and definition is a little
confusing and arguably it shouldn't do anything unless we also implement
gatherTransparentRegion. gatherTransparentRegion is already made public
through SurfaceView, and so exposing it properly on View seems like
a non disruptive option.

Bug: 179647628
Test: android.view.cts.ViewRootSyncTest
Change-Id: I72fb2744bc03cdd26d64547dbebeb12751a33a2a