Set mProximity to PROXIMITY_UNKNOWN on screen off

In the case where the screen was already off do to the proximity
sensor and the user presses the power button, we need to set
mProximity to PROXIMITY_UNKNOWN so that when the screen comes back
on and the proximity sensor is re-enabled it will be able to pick
up the change in proximity.

Change-Id: I53fc5d34597422546239bb71641355ca1aa5dea8
REF: CRACKLING-554