[pvr] adjust labels and refactoring for setting 'pvrplayback.startlast'

I adjust the current used labels *Minimised* and *Yes* for the setting *Continue last channel on startup* to *Background* and *Foreground*, so they will be more suitable in case a user choose one of this options.

Apart from the label changes, I also change the current implementation for the setting. I removed the filler method and add the option directly to the settings.xml, which saves some lines in PVRManager.
the param is only used for `CMediaSettings::Get().SetVideoStartWindowed(bMinimised);`. IMO preview does not reflect what's done here.