Ensure ProxCheck doesn't unregister ProxSensor

If the ProximitySensor passed into ProximityCheck is shared with other
clients, the ProximityCheck was causing the ProximitySensor to be
paused which, in turn, meant other clients would miss out on events.

Specifically, this manifested itself in AOD/Doze where one part of
Doze wanted the proximity sensor registered, but another part used
ProximityCheck with the same sensor, causing the sensor to be
paused. The screen would either not turn off when covered, or would
not turn back on when uncovered.

Fixes: 155365422
Test: atest SystemUITests and manual
Change-Id: I3c3faf79ba6d3915b0cfecec7861f6065ae689e8