Use new method of getting top layer FBO details from SkCanvas.

This avoids having to peer through multiple layers of private Skia
abstractions to get the FBO ID and size.

Also use framework-only SkCanvas method to get top layer bounds.

Test: Presubmit checks will test this change.

Bug: skia:11019

Change-Id: I62095162e80c4837485f0f89bd40372846303309