Avoid binding to providers if no apps are scanning

Regardless of manager scans (example: scans coming from sys ui or
settings). Concretely, this change will prevent apps that don't
integrate with MediaRouter2 from having remote volume control in
Settings (this is described in detail in linked bugs).

Bug: b/319604673
Test: Manually by casting using popular app that doesn't integrate MR2.
Test: atest CtsMediaHostTestCases CtsMediaBetterTogetherTestCases
Change-Id: I61f8a2de2fa617517527e775eccf38506a1b9a18