MediaRouter: Route setBluetoothA2dpOn() via MediaRouterService

The direct binder usage of AudioService.setBluetoothA2dpOn() is now
blocked by permission for security reasons.
This CL routes the usage to the MediaRouterService.

Bug: 181962322, Bug: 189176702
Test: GMM can play navigation sound when BT is connected
Change-Id: Ib13c224a0d412ef6d802eeaadd246649c819c26f