docs: Updates for picture-in-picture docs for N DP4

Adding two updates requested by eng for DP4: Devs should
use onStop/Start to pause and resume video playback instead
of onPause/Resume. Devs should manage audio focus properly
while in PIP mode to avoid interferring with other audio apps.

Bug: 29045184
Change-Id: Id3c302ff13b76c84bc3f4a51dd2dbf9688403669