[pvr] don't disable addons when database is reset

This is yet another attempt at fixing bug 15583. Earlier discussed here:

https://github.com/xbmc/xbmc/pull/4761 (introduced the change in behavior)
https://github.com/xbmc/xbmc/pull/5595 (fix, never accepted)
https://github.com/OpenELEC/OpenELEC.tv/pull/3673 (revert of #4761, in OpenELEC since about three weeks)

@opdenkamp AFAICT this introduces no regressions. Disabled addons remain disabled after the database is reset, enabled addons will remain enabled.