Prevent media button receivers targeting activities

This enforcement prevents a bypass of background activity launches. For
Android U, the enforcement just ignores the request and logs a warning.

This change is a backport of ag/23810567.

Test: Manually.
Bug: 272737196
Bug: 272024837
Change-Id: I2412633c0b3afda6776244c72043d0dd78a4c8a1
Merged-In: I07c2521f3d97f94e245a25bbb6f49f75cf54a808