[pvr] adjust labels for setting pvrplayback.startlast

This change the current used labels 'Yes' and 'Minimised' to 'Foreground' and 'Background', so it's easier to understand what's happen if you choose one of these options.
[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 bMinimised to be more reasonable
[pvr] refactor enum ChannelStartLast to reflect the setting
Merge pull request #4969 from xhaggi/pvr-refactor-setting-pvrplayback-startlast

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