Prevent media button receivers targeting activities

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

This change is a backport of ag/23810567.

Test: Manually.
Bug: 272737196
Bug: 272024837
Bug: 317203980
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:d455e21711c167223f7d0696809a4e411683182c)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:039ead905eb66d37154fe6bbf16b87863ad79265)
Merged-In: I2412633c0b3afda6776244c72043d0dd78a4c8a1
Change-Id: I2412633c0b3afda6776244c72043d0dd78a4c8a1