Fix NPE which occurs when MediaBrowserService.onLoadItem gives null

NPE happens in Android O and after because
MediaBrowserCompat.ItemCallback.onItemLoaded can take a null result
from Android O.

Test: Tested manually with MediaBrowserCompat which connects to
      framework MediaBrowserService. Checked that the null result
      can be sent successfully in Android O.
Change-Id: I882d0c775184925299401f849374cd8858ba3bdc
4 files changed