[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.
this was not added by me only changed see https://github.com/xbmc/xbmc/pull/5400/files#diff-819d3671c423a1cd31e0279c3eda6ee1L1094