Update the accessibility gesture recognition logic, using prior

assumptions about allowed gesture set to increase accuracy.

Test: Using gesture data from real users at
https://drive.google.com/corp/drive/folders/0BzgTMSNz7WvCNTExbGwtSTRWTzQ
measured accuracy via standalone app at
https://cs.corp.google.com/piper///depot/google3/java/com/google/android/apps/internal/accessibility/A11yGestures/
Also added unit test to check that synthetic gesture motions are recognized.

Change-Id: I06e61ec814ee52db7441c4998012c2df9910589e