For getHomeActivity() only return current user.

Previously getHomeActivity() returned the topmost home activity
independent of which user was currently running. That defeated the
purpose of the method. This fix returns the home activity of the
current user or null if one has not yet been created.

Also remove some cruft that accumulated.

Fixes bug 21055376.

Change-Id: Ic1d58129aedbe3624f8a9d12c05c84674687b0a4