Allow MediaRouter2Manager releasing the session created w/o request

When a RoutingSessionInfo is created without MediaRouter2(Manager)'s
request (i.e. created with REQUEST_ID_NONE by provider),
the MediaRouter2Manager cannot release the session.
This CL makes the manager be able to release those sessions.

Bug: 153832566
Test: Using platform-dev Support7Demos,
      1) Select any sample route (Variable volume~)
      2) Using SampleOutputSwitcher, select phone
      3) The sample route's screen disappears
      => Without this CL, the screen doesn't go away.
Change-Id: Ibf61ba52012c2785d2807288dcb338266156efd9