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

services/core/java/com/android/server/media/MediaSessionService.java

@@ -47,6 +47,7 @@ import android.os.Handler;
 import android.os.IBinder;
 import android.os.Message;
 import android.os.PowerManager;
+import android.os.Process;
 import android.os.RemoteException;
 import android.os.ResultReceiver;
 import android.os.ServiceManager;
@@ -697,6 +698,14 @@ public class MediaSessionService extends SystemService implements Monitor {
             final int uid = Binder.getCallingUid();
             final long token = Binder.clearCallingIdentity();
 
+            if (isGlobalPriorityActive() && uid != Process.SYSTEM_UID) {
+                // Prevent dispatching key event through reflection while the global priority
+                // session is active.
+                Slog.i(TAG, "Only the system can dispatch media key event "
+                        + "to the global priority session.");
+                return;
+            }
+
             try {
                 synchronized (mLock) {
                     MediaSessionRecord session = mPriorityStack