ModPower: optimized proximity wake up feature
Don't unregister sensor listener immediately but keep it running for
some time (MAX_PROXIMITY_TTL) keeping sensor state updated. This handles
situation when playful user keeps pressing power key repeatedly which
results in listener reg/unreg operation happening too fast in a row
which can sometimes confuse the sensor manager. We rather use current
sensor state coming from listener that's still alive instead of
registering a new listener.

Especially on Nexus 5, there seems to be some kind of a bug in sensor
HAL which can render prox sensor stuck in ON state forever, not sending
proper values at all (indicated in logcat as: W/qcom_sensors_hal(17970):
_hal_sensors_activate: Deactivating dirty sensor 31)