Return cached data for the Now Playing Queue

Similar to the need to cache the folder items, we need to cache the now
playing list since carkits will poll for the data in chunks. If we have
to go through the framework for the data every time this can be
incredibly slow causing lots of jenk.

Bug: 77879544
Test: "runtest bluetooth -c com.android.bluetooth.avrcp.MediaPlayerWrapperTest"
      Also test with BMW with a playlist of over 300 songs.

Change-Id: Ib2c8af797d3d87b79bbf386775506bcc06715a33