[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
You meant the owner who new the TizenSettingInfo object should call the setter to ensure the member is correctly initialized? If so, I think it is bad interface/class design. If a member is considered can be initialized with default value, we should that always.