When starting Exams, Notifications and Attempt logs can be out of sync

### Observed behavior

1. As a learner, start an exam, but do not do any of the questions
1. After some time, a Notification will be recorded for the exam with a status of "Started"
1. However, querying the class summary API, the 'exam learner status' for that exam will have a status of "NotStarted".

This can lead to an inconsistency, where the Notification will update the in-memory classSummary data and cause the dashboard and reports to show that 1 learner has "started" an exam, but if you were to refresh (and get the on-server class summary data without the updating notification), it will revert to showing 0 learners starting the exam.

### Expected behavior

Since, the UI intends to use Notifications to patch class summary in real time, the two notions of "Exam Started" should match to avoid situations like the on described above.


### User-facing consequences

inconsistent / fluctuating values in reports


### Steps to reproduce
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Precise steps that someone else can follow in order to see this behavior
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…

### Context

0.12.0 a 7
diff --git a/kolibri/plugins/coach/class_summary_api.py b/kolibri/plugins/coach/class_summary_api.py
index 3165251f01..b2227295db 100644
--- a/kolibri/plugins/coach/class_summary_api.py
+++ b/kolibri/plugins/coach/class_summary_api.py
@@ -108,9 +108,8 @@ class ExamStatusSerializer(serializers.ModelSerializer):
     def get_status(self, exam_log):
         if exam_log.closed:
             return COMPLETED
-        elif exam_log.attemptlogs.values_list("item").count() > 0:
+        else:
             return STARTED
-        return NOT_STARTED
 
     def get_num_correct(self, exam_log):
         return (
['kolibri/plugins/coach/class_summary_api.py']