Media player attempts to dismiss notifications when players removed.

NOTE: this is only a partial fix.  See bug for discussion.

(Cherry-picked from 725d67d8)

Bug: 169271494
Test: manual
Change-Id: I64db4cc618a812063c18a40ea4465aa4c520a3be
Merged-In: I64db4cc618a812063c18a40ea4465aa4c520a3be