[Tizen] Enable background-support attribute in settings element.

When "background-support" is enabled in config.xml application will not suspend when goes background.
* enable: Web Application's execution MUST NOT be suspended when it is put into background
* disable: Web Application's execution MUST be suspended when it is put into
  background

BUG=XWALK-1148
Think it's useless, the handler will make sure to initialize the member. And all other members also did like this.