Improve scaling vs pan in screen magnifier.

1. Due to frequent changes of the behavior of ScaleGestureDetector
   this patch rolls in a gesture detector used for changing the
   screen magnification level. It has an improved algorithm which
   uses the diameter of min circle around the points as the span, the
   center of this circle as the focal point, and the average slop
   of the lines from each pointer to the center to determine the
   angle of the diameter used when computing the span x and y.

Change-Id: I5cee8dba84032a0702016b8f9632f78139024bbe