Handle KEYCODE_SOFT_SLEEP from Ungaze.

Needed for Ungaze to trigger "soft sleep" (respecting wake locks); operates by
sending new KEYCODE_SOFT_SLEEP to PhoneWindowManager, which calls
PowerManagerService's new method setUserInactiveOverride (thereby
causing immediate sleep, modulo wakelocks, upon next iteration of
PowerManagerService's main loop).

BUG: b/23589870
Change-Id: I24a96bd6db8ff28674c907f2898e49c4f6140209