MediaSessionService: Notify volume key long-presses when the screen is off

When the screen is off, MediaSessionService doesn't get the volume key
long-press events from the PhoneWindowManager because PhoneWindowManager
intercepts them and only sends initial down and up events.
This is intended because users cannot see the safety warning dialog
which warns users about too loud volume.

This CL assumes that volume key is long-pressed if the key up doesn't
happen after the initial down within the long-press timeout, and notify
them with IOnVolumeKeyLongPressListener.

Bug: 35328001
Test: Manual test
Change-Id: I3b5aec4c2dd66a181bf41786f1417cc65874356c