MediaSessionService: Nullify the media button sesison when it's removed

When the media button sesison is removed, we'd better not pick the
alternative media button session within the app because the alternative
media session might be a dummy which isn't able to handle media keys.

Test: Run the CtsMediaTest (MediaSessionTest and MediaSessionManagerTest)
  and the manual test (Launch a media app which creates two media
  sessions, and confirmed that another media session doesn't become the
  media button session when the media button session is removed)

Change-Id: I376a34ee4b5d2344007594134c40b135b2c5a1bf