Document the Gesture Detector constants.
Remove kEdgeSlop since it's obsolete on Android.
Add a test that verifies that when the first tap is canceled, the second
tap can become the first tap of a subsequent two-tap sequence.
Don't use a timeout to cancel tap tracking. Track only one primary pointer
and ignore non-primary pointers. Update tests to reflect desired behaviors.
Fixes#1779, #1780, #1781.
