Added Loader.onContentChanged() which is called by ForceLoadContentObserver
when it detects a change, rather than forceLoad().
By default onContentChanged() just calls forceLoad(), so there's no change
in behavior.
This is useful when a subclass wants to perform custom operations upon
data chantes. For example, a subclass may want to limit the number
of automatic requeries per second.
Change-Id: I493dac3f4f1a75b056d2c7065336ea9252dbf424
ec37c88570