Add streamlined config-based path for no volume key sounds

This lets us avoid a Binder roundtrip to the system process during
volume changes.  Previously volume change would be staged at key-down,
then applied at key up in tandem with playback of the sonic feedback
about volume key presses.  The reason for this two-stage handling was
to defer playback of the sound [at the target volume] while the volume
key was being held for repeat.

On some devices volume is always sent as key down/up pairs rather than
down-repeat-up sequences. On these devices it is more efficient to
apply the new volume immediately during down handling, and have the
up handling be a no-op.  This CL adds a configuration resource item
selecting this new fast path.

Bug 6433943

Change-Id: Icffa56e958243b841d514e2fe4609ba3a7b20f14