Invalid access to AccessibilityInteractionController

1. The Interaction connection checks if the ViewAncestor
   is valid and posts a message to be processed on the UI
   thread. The code in the UI thread did not check if
   the mView was valid. Added this check.

2. Added a faster sync mechanism in the test for
   getting the source of the last access event.

Change-Id: I9982b6592f5f44a68a67df40f37c063a3ba62993