MediaSessionManager: Split abstract class Callback into interfaces

This CL splits the MediaSessionManager.Callback (abstract class) with
OnMediaKeyEventDispatchedListener and
OnMediaKeyEventSessionChangedListener (interfaces with default methods).

It's to follow the API guideline and also to help the Bluetooth for
minally registering callbacks. (The Bluetooth currently only overrides
onAddressedPlayerChanged)

Bug: 138648566
Test: Build and run
Change-Id: I3032abaed5007baf941ef55166355191d8c47db3