When the controller of the `MediaNotificationManager` is disconnected,
the session is removed from the service without checking whether the
session hasn't already been removed. This caused flakiness in `MediaSessionServiceTest.addSession()`.
Because there is a public API `MediaSessionService.removeSession()`,
the controller can't make an assumption whether the session is still
contained in the service when being disconnected.
#minor-release
PiperOrigin-RevId: 572568350