Prevent use of disabled ConversationCursors

1) Change ConversationPositionTracker to get the ref to the
   ConversationCursor from the AbstractActivityController
2) Make sure to notify the position tracker about changes to the cursor
   in onLoadFinished

Bug: 6994143
Change-Id: I1d1e38852bb6cec1bc254b099c52f04bbe3af99d
3 files changed