[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
Merge pull request #2597 from takethathe/background_support

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