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.