Take touch point size into account during the detection of scaling
gestures. Average together the major/minor axes of the size.
Add a config resource for tuning the minimum span requred to begin
(or end) a scale. This may be altered in a device-specific overlay
for devices that deviate too far from their assigned density bucket.
Change-Id: I2986888e9427a7e4cb56717d59c4fa7858ba9ca7
828e56ea6e