Remove unpaired AccessibilityEvent

After a failed gesture, TouchExplorer was trying to announce
the transition to touch exploration mode. Since TouchExplorer
doesn't enter this mode, stop notifying a11y services.

Besides, don't announce the end of the touch interaction if users
don't lift the fingers.

Bug: 118724767
Test: A11y CTS & unit test
Change-Id: I0320c47ac82e5371ef20c5bd16ef2d059ebad6f6