Fix the failure of transferring to unselected system route

If we firstly pair the phone to a bluetooth speaker which is
the selected route, then we cast to a remote device. At this
moment, transferring from the remote device back to the
builtin speaker will fail. The fix is to first transfer from
the selected bluetooth system route to the builtin speaker
route, and then release the remote route.

Flag: com.android.media.flags.fix_transfer_from_user_route_to_unselected_system_route
Test: presubmit and manually tested with demo app
Bug: b/398527989
Fix: b/398527989
Change-Id: I1622347bc75290ea08d7cf376983dfee7f3ccde8