Add Activity methods for icons and labels.

New Activity methods setRecentsLabel(CharSequence) and
setRecentsIcon(Bitmap) have been added. The topmost
activity with either of these set will be returned in
RecentTaskInfo members activityLabel and activityIcon.

Fixes bug 13562992.

Change-Id: Ic15d1d27b733b892a2a940063b105ac48f1ffee5