[Tizen] Tune up app-widget element parsing in its manifest handler

Tizen 2.2 WRT Core Spec:
 - does not classify as invalid, widgets with:
   - more than one primary app-widgets,
   - no primary app-widget,
 - does specify that box-icon element is optional.

This patch removes unnecessary validation of primary attribute
in app-widget element and fixes parsing of box-icon element.

Link to the spec:
https://source.tizen.org/sites/default/files/page/tizen-2.2-wrt-core-spec.pdf