MediaController: Remove dispatching key event as system service APIs

This removes hidden API usages from the fwk to apex, which would be
prohibited.

With this API, the dispatching logic is changed as follows for sending
key events directly to MediaSessionRecord through MediaSessionService.

Before:
    PhoneWindow
    -> MediaController.dispatch***AsSystemService(key)
    -> MediaSessionRecord

After:
    PhoneWindow
    -> MediaSessionManager.dispatch**AsSystemService(sessionToken, key)
    -> MediaSessionService
    -> MediaSessionRecord

Bug: 123485698
Test: Build, flash and injected some key events
Change-Id: I3e0bca7565ae2f142c3389322d867700646d5038