AVRCP: fix single item queue Now Playing issues

When queried (and in track change notifications) we previously returned
an item ID not allowed by the spec (0xFFFFFFFFFFFFFFFF) to a controller
when presenting a "single item queue" because the player doesn't provide
one.

Refactor the code so that the single item queue is persisted and send
NOW_PLAYING_CONTENT_CHANGED when the single item queue updates.

Tweak debugging of mediacontrollers for easier player debugging.

Test: check media info on honda carkit w/fm.player app
Bug: 62288178
Change-Id: Ic37d0c4c837436e102e9642e1f8e36e17e0dc254
(cherry picked from commit 3a80469f)