docs: Adding documentation for "gregarious mode" (background playback)

for Android TV.

Formerly this was cherry-picked from Andy S's CL: ag/732685
Into a CL of mine: ag/838200
However, I somehow managed to create a CL that could not be merged
(it complains about "this change depends on changes that are not ready").
It's likely because I tried to revert an erroneous change Andy
made via git commands, and probably did something wrong there.

Anyway, I've copied my former CL exactly, but without cherry-picking,
so it's now a new independent CL that should be mergeable without
problem.

Bug: 21209207
Change-Id: Ib5c6e031f9a3e43ab80b05bfe1edbfd81d83e54d