Remove obsolete method in IMediaBrowserServiceCallbacks

It removes onLoadChildren because onLoadChildren is not used anymore
that is identical to onLoadChildrenWithOptions with null options. It
also renames onLoadChildrenWithOptions to onLoadChildren for simpler
name.

Test: atest CtsMediaTestCases:android.media.cts.MediaBrowserTest \
  CtsMediaTestCases:android.media.cts.MediaBrowserServiceTest
Change-Id: I6aebd812249cd3a717aa6d2f48e4a40601ef1063