Browser: Fix browser force close

Issue: Cursor was accessed by handler thread
       when it was closed. During Orientation change,
       BrowserBookmarksPage#onDestroyView is called
       which clears all the loaders [and cursors].
       Simultaneously ThreadedCursorAdapter#loadRowObject
       is getting called which then calls
       BrowserBookmarksAdapter#getRowObject
       where closed cursor is accessed and force close happens.

Fix: Synchronized cursors wrt main thread and handler thread.
     Checked mCursorAdapter for invalid conditions

CRs-Fixed: 576176
Change-Id: I9f16c2f1a55f35e8256bbf56afce3538850f9d42
(cherry picked from commit bd90a0d266b3ed7818e98ee88ed3e3ca9c9c57fa)