Fix deadlock in MediaRouterService

The deadlock was introduced by the previously merged CL
"2942e4b: MediaRouter: Route setBluetoothA2dpOn() 
  via MediaRouterService".

Bug: 181962322, Bug: 206160544
Change-Id: I3936b8345e8df20a51400a9083d46872e6801568
Test: Builds successfully

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

@@ -100,6 +100,7 @@ public final class MediaRouterService extends IMediaRouterService.Stub
 
     // State guarded by mLock.
     private final Object mLock = new Object();
+
     private final SparseArray<UserRecord> mUserRecords = new SparseArray<>();
     private final ArrayMap<IBinder, ClientRecord> mAllClientRecords = new ArrayMap<>();
     private int mCurrentUserId = -1;
@@ -345,9 +346,7 @@ public final class MediaRouterService extends IMediaRouterService.Stub
 
         final long token = Binder.clearCallingIdentity();
         try {
-            synchronized (mLock) {
-                mAudioService.setBluetoothA2dpOn(on);
-            }
+            mAudioService.setBluetoothA2dpOn(on);
         } catch (RemoteException ex) {
             Slog.w(TAG, "RemoteException while calling setBluetoothA2dpOn. on=" + on);
         } finally {