stats: Fix generation for trigger

Created by: sinkap

The trigger.generate function needs to return the value for the valid indices replaced with the trigger value This value replacement was being handled in Aggregator which was accessing a private member from the Trigger class. Removing the _ for making it public was possibly a bad design choice to make pylint happy.

Signed-off-by: Kapileshwar Singh kapileshwar.singh@arm.com