Optimize ProximityInfo#computeNearestNeighbors()

This cuts the time spent in computeNearestNeigbors by
a factor of 6 to 20, with an average at about 8.
This method representing about 25% of the total time
spent when switching keyboards, this change is a
20% improvement in layout switching time.

Bug: 8689779
Change-Id: Icb465f6010b317c7156057891c3341e51a198688