House all AVRCP metadata in custom objects instead of MediaMetadata

Lots of metadata comes over AVRCP and we're only saving a subset of it
into Media Framework objects (mostly because the framework objects don't
have anything in them that relates well to the AVRCP metadata). This
change migrates us to an AvrcpItem that can hold any extra metadata we
need and knows how to convert itself into the correct framework objects
such that we can still share the data with our MediaBrowserService.

Bug: b/132812696
Test: Build, Flash, interopt tests with devices, atest
Change-Id: I86f78de60bbe95229f63c5d282494453f920c6a6