Use alternative prox sensor when specified.

This is a manual upstream merge of http://ag/10320285 from Q to R.
It does not merge cleanly since the usage of prox sensors has been
consolidated in R.

With this change, devices are able to specify an alternative proximity
sensor via R.string.proximity_sensor_type, along with a threshold
value below which the proximity sensor is considered "near".

Prior to this, the R.string.doze_brightness_sensor_type value was
serving double duty for this purpose, but, for at least some devices,
we want to have a different brightness sensor vs proximity sensor.

Bug: 149420648
Test: manual
Change-Id: Ifd2006cce1c7a653fd67a7a33b63c5f638384bab