InCallUI: Don't force enable the prox sensor for incoming calls

The proximity sensor should be enabled for incoming calls only if the
proximity check on wake feature is enabled. A device with no proximity
check on wake support does not have PROXIMITY_ON_WAKE set, so getInt()
returns the default value (enabled) and we incorrectly enable the
proximity sensor for incoming calls. Check whether the proximity check
on wake feature is supported or not and never enable the proximity
sensor for incoming calls if it isn't.

Also, since there's an overlay to specify the default value for the
proximity check on wake, use that instead of the hard-coded "enabled".

Change-Id: I17598c24d1908107927881fc31a701457094c61e