Prevent callbacks being called before the constructor

BluetoothRouteProvider requires a listener in the constructor, but
the listener method can be called before the constructor finished.
This CL makes the listemer mether be called after the constructor
finished.

Bug: 149751047
Test: pass MediaRouter2 tests
Change-Id: I3fae3f64253f3f7d3417f71bae76e2f483db3e69