Select correct system audio route in updateAudioRoutes

While update audio routes, MediaRouter had an access to the variable
of MediaRouterService.mGlobalBluetoothA2dpOn, which was updated in
the callback of AudioRoutesObserver.dispatchAudioRoutesChanged().
However, since updateAudioRoutes() was also called by the same
callback, mGlobalBluetoothA2dpOn could be used in updateAudioRoutes()
before its value was updated.

Bug: 65629167
Test: passed MediaRouterTest
Test: Manually confirmed that the issue is fixed with this CL
Change-Id: Id0034996a51a6d8d1e8bd9d2c6ca386aabbb8baf