Recommendation is to provide a 5% overscan margin -- updating docs
to reflect this. Updated example diagram (manually edited, image
source not available). Added guidance on working with margins and
layout. Updated sample layout to include background root layout and
nested layout with overscan safe margin (tested with test app).
Bug: 18550276
Change-Id: I71b83b9a348385d68a7aacdce17f51a93227694d
cc40d8cd86