fix(framework): CSS Vars in Static Styles work on IE11

[vladitasev]

The createComponentStyleTag method did not take into account Static Area CSS and did not inject it in the head. Instead, this CSS went to the ui5-static-area-item like on normal browsers. However, it wasn't processed there so CSS vars were not replaced.

The fix:

• Do not inject any CSS in _updateFragment for IE11 (if shadyDOM is true).
• Inject the static area CSS along with the normal CSS in createComponentStyleTag instead, where it will be processed like the regular CSS
• Finally, add a static getter for static area CSS in UI5Element.js to ensure it's always a string.