Prevent media button receivers targeting activities

This enforcement prevents a bypass of background activity launches.

Bug: 272737196
Test: atest MediaSessionHostTest MediaSessionTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:edf219ab08fdf5390aac6dd18fcdab76daa3488a)
Change-Id: I07c2521f3d97f94e245a25bbb6f49f75cf54a808