DO NOT MERGE Check caller for sending media key to telephony service

Prevent sending media key events from the non-system app to the
telephony service through the AudioManager.dispatchMediaKeyEvent()
or sending media key broadcast directly.

Bug: 29833954
Tested: Installed malicious apps and confirmed that they don't work.
Tested: Run CtsTelecomTestCases and CtsMediaTestCases
Change-Id: I2a9e78196ba7455324e485f098f095d03b47ee15