DO NOT MERGE Check caller for sending media key to global priority session

DO NOT MERGE Check caller for sending media key to global priority session

Prevent sending media key events from the non-system app to the global
priority session through the MediaSessionManager.dispatchMediaKeyEvent().
Note that any app can use the API indirectly with
the public API AudioManager.dispatchMediaKeyEvent().

Bug: 29833954
Tested: Installed malicious apps and confirmed that they don't work.
Tested: Run CtsTelecomTestCases and CtsMediaTestCases
Change-Id: I2a9e78196ba7455324e485f098f095d03b47ee15
(cherry picked from commit c282c4f106bea5c5375fdb6062ffd4fdcf8563a7)