Fix infinate initial load.

The UserChanges table is a join table comprising of both the Users and
 the Changes tables. When either the Users or the Changes table is
 modified, queries of the UserChanges table may need to refresh.
This notifies that the UserChanges table has been modified when either
 one of its subtables are modified.

Register a content observer to intercept changes and re-broadcast them
 in the UserChanges table.

Change-Id: I329d08b79e82a00365d8cd1e0f82baaeb704d80b