Clean up CursorWindow lifetime.

Bug: 5332296

Removed dead code in SQLiteCursor related to the use of a background
query thread.  This code could result in CursorWindows being modified
concurrently or used after free.  This code is broken, unused and
is just in the way.

Added comments to explain how CursorWindow ownership is
supposed to work for AbstractWindowedCursors.  (There are still cases
where cursor windows get dropped on the floor without being closed.
Those will be taken care of in a subsequent patch.)

Cleaned up SQLiteQuery.fillWindow to eliminate duplicate code and
remove bits that were only needed for background loading, like
returning -1.

Change-Id: I03e8e2e73ff0c11df76d63f57df4c5ada06ae1cb