[DSRM] Fix the metrics wrong duration

- add back start time when data stall happen first time
- add the unit test for this case, ensure the start time not zero.

Bug: 260166714
Test: Manual test passed. See the correct time in log.
      Telephony sanity test passed. Result: b/269436482.

Change-Id: Ia33e05eadc2e96a1b2b78c488af4efcb9811065e
Merged-In: Ia33e05eadc2e96a1b2b78c488af4efcb9811065e