Adding a method called when detecting content change.

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