AVRCP: Passthrough through MediaSessionService

Send passthrough keys through MediaSessionService instead of trying to
interpret them ourselves, and log who they get dispatched to.

Remove the hack around down fast-forward / rewind.

This should result in a lot less confusion about which app is addressed
when the user is sending a command (such as at car connection time).

Test: play / skip, switch app on phone, reboot, start from carkit, dumpsys logs
Bug: 33828042
Bug: 37476911

Change-Id: I8c8c40cb3792254a3720f64707e67fdcc940edaa