Inverts behavior of double touch drag gesture

Per Android guidelines, double touch drag up should decrease content scale and double touch drag down should increase content scale.

This is also the behavior in the official Google Maps Android application.

http://developer.android.com/design/patterns/gestures.html
Adds double tap to zoom
invert tilt gesture
make double tap zoom to screen location
fix double-tap zoom tests
Merge branch 'double-tap-to-zoom'