[pvr] fix: last watched timestamps won't update after channel switch

As we only update the last watched timestamp of the current playing channel before a successful channel switch is performed we sometimes continue the wrong last watched channel and group.

@opdenkamp or @Jalle19 for a second look.
does this even work? you create a new copy of a channel here and update the last watched time in there. that won't be picked up until xbmc is restarted. which is when this is normally used, but say someone wants to use the last watched status for something else, then this code won't do what you expect it to do.