Clear preferred feature when unregistering MediaRouter2

MediaRouter2Managers' preferred feature didn't get cleared even when the
MediaRouter2 is unregistered/died. As a result, when an app is
reinstalled/relaunched, the routes for the previous process are still
shown in the MediaRouter2Manager.

This CL fixes the problem.

Bug: 158824569
Test: Passed CTS and mediaroutertest
      Also tested with reinstalling support7Demos.
Change-Id: Ib5d84c41862177af32931e0d69b02f461105bba6