Treat playlists like first class media files

Make PlaylistsColumns inherit from MediaColumns.
Currently, ContentResolver#openFileDescriptor is broken if used with
Playlists content URI, in addition, we cannot query playlists from the
playlist view because the columns there don't match the columns defined
in PlaylistsColumns. Now that we're advising app developers to use
ContentResolver for their storage needs, we need to make sure the flow
works properly.

This was discovered when addressing a bug filed by 1P app, YouTubeMusic

Test: build
Fix: 150593090
Bug: 150324585
Change-Id: I9950510df65c59a6f74c90c2ca28d281bb08f5b6