The active window for accessibilitiy incorrectly tracked.

1. The active window for accessibility purposes is the either the
   window the user is touching or the window that has input focus. We
   were using the touch exploration gesture end event to figure
   when the user stops touching the screen so we can set the active
   window to the input focused one. However, we do not send such
   gesture end if the user does not touch explore. If the user only
   taps we do not consider this touch exploring. We now have dedicated
   accessibility events for first and last touch and this change uses
   them as a guide when to update the active window.

bug:6523219

Change-Id: I6262c0c5f408b02dbaa127664e4b426935d7f81f