Dialer: Fix proximity on wake handling

*) http://review.cyanogenmod.org/#/c/167122/ introduced an issue
   where the screen would never turn off during a call if the system
   proximity on wake setting was turned off.

*) Fix this by making sure the proximity_on_wake setting is only
   used when we have an incoming call.

Change-Id: I49f8759112c4f9bfe41325dd3329eb2d1902231f