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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