[Tizen][Runtime Model] Using the xwalk logo image when icons are not given in manifest.json

The application button with default xwalk logo image will be placed on
home screen in Tizen if the icons field is not given in manifest.json.

BUG=https://crosswalk-project.org/jira/browse/XWALK-124
Merge pull request #1140 from takethathe/icon

[Tizen][Runtime Model] Using the xwalk logo image when icons are not given in manifest.json