Add missing passive discovery flag

MediaProjectionManagerService had an active media callback which
was causing a service to be bound 100% of the time.  Adding a
passive flag makes it only observe events, and allow the service
to only be bound when needed by apps requesting active discovery.

Bug: 18042409
Bug: 17969854
Change-Id: I1bfa6609e2aa507ee2ce227de50f0e5ae951e000