[pvr] remove unnecessary filler method for setting 'pvrplayback.startlast'

This removes the filler method for setting 'pvrplayback.startlast' and add options directly to the settings.xml.
[pvr] refactor: change method parameter bPreview to bPlayInBackground to be more reasonable
[pvr] refactor enum ChannelStartLast to reflect the setting
[pvr] refactor: rename setting 'pvrplayback.startlast' to 'pvrplayback.continuelast'