[application] - remove m_bInBackground and its setter/getter and use

SetRenderGUI instead.

As discussed here:

I tested music background playback and didn't spot any differences. So i think its good.

https://github.com/xbmc/xbmc/commit/5b60bac7a11675aefe2532e0eb06b61d7efc7f91#commitcomment-6167344