Fix trigger-task reporting when finishing a task

Was reporting the activity, but should be reporting task. This is
when the last activity finishes in a task.

Bug: 169035022
Test: ActivityRecordTests#testFinishActivityIfPossible_lastInTaskRequestsTransitionWithTrigger
Change-Id: I930c38716a795397bb888a1e0b7d8c7190ad2fe9