Fix Timer unittest

* Never set promise more than once
* To verify running task cannot be cancelled, make sure Cancel() is
  called after callback starts
* Remove a stress test case. Sometimes there is unexpected event causing
  deviation.

Test: run Timer test for 5000 times
Bug: 116081383
Change-Id: If99e4958289e9b6aaabc51b15ce4ac035a3ff1e9