Fix class cast issue in AddItemActivity

AddItemActivity is a standalone activity. There is no Launcher activity
instance. Avoid using LauncherAppWidgetHostView in WidgetCell by
replacing it NavigableAppWidgetHostView instead

Test: Check preview layouts are correctly rendered in widgets picker
      and pin widget dialog. Also test drag and drop to verify the
      local color extraction still works when dragging a preview layout
      from widgets picker to home screen.
Fix: 193765783
Change-Id: I3a734a54941257cebcd7b7f6a4c2c9cc764c5c12