New user interface sound effects:

 - Low battery. (http://b/2320026)
 - Dock/undock events.
 - Keyguard lock/unlock events.

New system settings have been created to turn these on/off
and to specify the relevant sound files.

[Production notes: The provided low battery sound and dock
sounds were synthesized; the lock screen sounds are
processed samples of a ballpoint pen click mechanism.]

Bug: 2320026
Change-Id: I374285b0f94f59c7555bb8816580f5a8c802e90d