[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.
iirc, this param also controls whether or not to actually switch when delayed channel switch is used