DisplayConfig: Add flag-protected option to select Fusion Proximity sensor.

This adds to capability to add a second Proximity Sensor
configuration that is selected based on an aconfig flag.

In detail:

1) Adds a new flag "useFusionProxSensor" that controls whether or not
   to use a Fusion Proximity sensor if configured.

2) Adds a new attribute "featureFlag" to the sensorDetails XML
   element that marks the element to be used if a corresponding
   aconfig flag is set.

3) Allows up to two "proxSensor" elements to be given instead of one.
   This allows to provide two different Proximity sensor
   configurations, one to be used if both the useFusionProxSensor
   flag and featureFlag attribute are set and matching, and another
   if not.

Bug: 306203895
Test: Pushing different XML config files, confirming via dumpsys and
      manual testing.

Change-Id: Icb7a1c93d38385b48ba27d9747cae9cd36c4dc03