docs: New section explaining how to set the custom launch color of TV app

NOTE: Cherry picked from:
https://googleplex-android-review.git.corp.google.com/#/c/732032/

Contains updates incorporating all feedback from Andy's original CL.
Updated with suggestion from Andrew.

Bug: 21274505
Change-Id: If43ba81e770a39239108b8fa9731c2b4df3a9ae6