[IMP] mrp: time tracking consider now as end date

When changing the duration on a time-tracking entry on a workorder, set
the end time to now and calculate the start time by detracting the
duration, instead of the other way around.

Task: 3456827 (nr 35)
Part-of: odoo/odoo#137535

addons/mrp/models/mrp_workcenter.py

@@ -418,7 +418,7 @@ class MrpWorkcenterProductivity(models.Model):
 
     @api.onchange('duration')
     def _duration_changed(self):
-        self.date_end = self.date_start + timedelta(minutes=self.duration)
+        self.date_start = self.date_end - timedelta(minutes=self.duration)
         self._loss_type_change()
 
     @api.onchange('date_start')