Moving LoaderTask static

Will move it to a separate file in a followup cl.
This simplifies dependencies between LauncherModel and LoaderTask which
and making it easier to start the loader before Launcher activity is
created (as the Callbacks in LauncherModel can change while loader is running).

Bug: 34112546
Bug: 37616877
Change-Id: Ie9619c6b0de0e3eb60657c04ae1b58d946c829e9