Clean up behavior of type arguments for MediaRouter#getSelectedRoute

MediaRouter's policy so far has been around a single selected route,
but when route types are entirely orthogonal this should not be the
case. However we still don't want to get into a situation where we
have multiple, very different routes selected for different types at
the same time, we still want to have more of an element of
predictability.

Behavior of getSelectedRoute is now:

* If the selected route matches at least one type with the requested
  type flags, it is still considered selected for that request.

* If the caller specifically requested the selected user route and the
  currently selected route is not a user route, return null.

* If the requested type flags do not match any types with the selected
  route, return the default system route.

Note that this is "any" behavior instead of "all" - this matches
existing usage of the method. We may consider adding an "all" variant
later on.

Bug 7588042

Change-Id: I3a79d8153ca6b882fd3ef6b9b1de8f538873dec2