[Media ML] Refactor APIs for dispatching key events

There are currently two APIs with the same name:
dispatchMediaKeyEventAsSystemService(KeyEvent), and
dispatchMediaKeyEventAsSystemService(KeyEvent, MediaSession.Token),
but their internal implementations and return types do not overlap.

Instead rename the second API to
dispatchMediaKeyEventToSessionAsSystemService(...) to avoid
associating the two APIs.

The same logic is applied to dispatchVolumeKeyEventAsSystemService.

Bug: 166262380
Test: build
Change-Id: I20f7c6d4dd97eff6625307cf9bbc14ff824c3286