MediaRouter2: Release controller when transfer result arrives

Previously when MediaRouter2#transferTo(route) is called, it directly
released current controller. This CL changes the release timing to
when the actual transfer result comes.

Also, this CL prevents a RoutingController instance from being passed
both to:
 - As 'oldController' in onTransfer()
 - As 'controller' in onStop()

These works make TransferCallback easy to use: when onStop() is called,
app can just stop playing media.

Bug: 154680146
Test: Passed CTS / Checked logs in onTransfer/onStop
      with output switcher
Change-Id: I4449021f98ca887252e05398356df9b53592c145