AVRCP: Fix NowPlayingList looping

When a controller requested GetFolderItems, we would sometimes send a
NowPlayingListChanged before responding to GetFolderItems, prompting a
new GetFolderItems request.  This put us in an infinite loop.

Test: connect to BMW and note it actually works
Bug: 62775732

Change-Id: Idc669a03f71ed9ec6b211bb5c33fc98548cc9f85
(cherry picked from commit d65422f9)