DO NOT MERGE

Use the common ScaleGestureDetector to detect the
multi-touch motion.

Check both supportZoom and getBuiltInZoomControls
to decide whether enable the multi-touch behavior.
This should the new pinch behavior only replace the
old +/-.

Fix http://b/issue?id=2363260

Only update textWrapScale if it is different. This
should fix the performance decrease caused by initial
multi-touch code.

Fix http://b/issue?id=2371694