FMRadio: (almost) drop system UID

* Add FMRadio app to media group to allow maintainers to start
  changing their trees
* Add various fixes for running with less permissions (previously, we
  got said permissions from other apps in the system UID)
* Do all preparations so the sharedUid can be dropped without breakage
* ACCESS_BROADCAST_RADIO adds us to media group which allows us to
  access /dev/fm without system uid
* ACCESS_FM_RADIO is no-op

Change-Id: Ib8847a6104cab9d8dac6e22c82104fd780276052