docs: Fixing doc issue with Android TV overscan margin recommendations.

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