Prevent premature task abortions, Store timestamp first (EXPOSUREAPP-3878) (#1668)

* do not prematurely abort diagnosis keys task, if there is no last server fetch time; before anything else store time stamp after successfully submitting keys to enf

* comments and logs

* We need to reset the tracked exposure detections on app reset.

* Make sure that all flow preferences reset when we use the app reset function.

* Add workaround for EncryptedSharedPreferences, it's `clear` behavior is not like a normal sharedpreference,
as no listeners are called when keys are removed.

* Fix failing test.

Co-authored-by: Matthias Urhahn <matthias.urhahn@sap.com>