Lock screen should be kept on while it is touch explored.

1. The lock screen goes off after a certain timeout but a blind
   user performing touch exploration of that screen needs more
   time to find controls and interact with them. Therfore, we
   reset the lock timeout by poking the wake lock on the last
   hover event so the lock timeout will start ticking after
   the user has stopped interacting with the lock screen.

bug:5133879

Change-Id: I340d2a820e90f6fd63a45d65a5457ce6e570189c