Restore the selected route when an app starts to make sound

Limitation: When two apps make sound simultaneously while one uses a BT
route and the other uses the default route, switching activities between
the two different apps will not change the BT A2DP status until one of
the apps changes its playback status to inactive.

Bug: 32958908, Bug: 35567695, Bug: 33186346
Test: Passed MediaRouter/MediaSession test, and manually varified that
      the b/33186346 is fixed.
Change-Id: I1da97dcb74fa2095425b311ad62cdd00f8118cea