Submission Improvement (#438)

* Warn Others Fragment
- control button enabled and progressbar visible via databinding & formatter
- remove requestPermissionToShareKeys() from onResume which causes multiple submissions to be triggered
- remove unnecessary state submissionRequested & submissionFailed
- add rudimentary error handling to onFailure of exposure notification API <- needs to be confirmed!!

SubmissionViewModel
- inline executeRequestWithStateForEvent
- changed submissionState from Event<ApiRequestState> to ApiRequestState to enable use of formatter
- use postValue to update from background thread

misc:
- remove unused formatShowTanCharacterError method

* Warn Others
- adjust onFailure handling