Activity selections not always persisted by AcitivtyChooserView.

Activity selections not always persisted by AcitivtyChooserView.

1. ActivityChooserModel was delegating the rsponsibility for reading
   and persisting data to its clients i.e. ActivityChooserView and
   the latter was persisting on detach from window. However, it is
   not guaranteed that this callback will be received leading to
   lack of presistence sometimes. Now the model is responsible for
   loading and persisting thus voiding the posisbility a misbehaved
   client to cause data loss.

bug:5061638

Change-Id: Ia2b7bb659f7b9abe8869c94b3eaa726ddd4e4fad