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