sensorservice: Refactor tracking logic for the Proximity sensor.

This simplifies the code that tracks if the proximity sensor is
active by removing all counting of clients and simply checking the
sensor's isSensorActive() method and reporting changes to it.

Bug: b/202328704
Tests: See b/202328704#comment9 for tests done.
Change-Id: Idd72b82ed0fd5ee089e795ab4b233241350979c2