Updating PIP preview docs to mention that devs need to
use MediaSession.setCallback to get input events while in
PIP mode, and not MediaSession.setMediaButtonReceiver.
Note: Changes also include changes made previously for
DP4 in a different branch that for some reason were not
auto-merged into mnc-io-docs, so including these changes
as well to ensure they don't get overwritten.
Bug: 29215189
Change-Id: Iab51c26ebd1f1403633eb9a1616d43690173446c
83b25f7c46