[pvr] fix selecting last playing group 'All TV/Radio channels'

As the last watched property of the internal 'All TV/Radio channels' group never gets updated if we fetch it from the database it's impossible to select this group on startup as the last played group.

@FernetMenta  this fix the root cause of http://trac.xbmc.org/ticket/15397 where some users reported that the continue last playing channel option won't work. We attempted to fix it with #5526 but that fixed only the half of the issue. 

@opdenkamp or @Jalle19 for a quick look.