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
1 file changed