docs: Fixing MediaSession note in PIP preview docs

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