sensorservice: ProximitySensor needs to report correct sensor state.

Calls to ProximitySensor::activate() may repeatedly set enabled=true.
The current logic doesn't account for that. In order to correctly
report the current sensor state, simply use
mSensorDevice.isSensorActive() instead.

Bug: 194878856
Test: 1) Reproduced problem and verified mIsProxActive in "dumpsys
         display" contains the correct value and no more 60/90Hz
         flicker.
      2) Adding local debugs, then mixing making phone calls with
         additional "sensor_test sample -s8 -n5" commands to create
         several Prox clients, and verifying logged flags and
         reference counts.
Change-Id: I2342844ff4e6301a6b9ac8a33dc3e6047fca83ad