AOD: Add hysteresis to pausing the display

Delays pausing the display by several seconds to avoid
flickering when the prox is only covered transiently.

Also adds cooldown to the proximity sensor, such that when
it toggles too often it gets disabled for a while.

Bug: 62292293
Test: cover prox for less than 10s, observe display stays on. Cover for more, observe display turns off.
Change-Id: Ifa407b84760fc299fbbcfa92d9e942e0093c4b73