Repository for One Time Passwords(EXPOSUREAPP-4857) (#2232)

* Skeleton classes for SafetyNet and EventSurveys.

* Fix LINTs, test compile error.

* Remove lint breaking TODOs.

* Add SafetyNet requirement validation call to the attestation interface.

* Don't return salt/nonce, just offer the caller directly the PPACAndroid protobuf class.

* Add documentation, fix test build error.

* Add documentation, fix test build error.

* + otp repo

* unit test

* requested changes

* + timestamp

* using gson

* mockFlowPreference

* https://github.com/corona-warn-app/cwa-app-android/pull/2232#discussion_r567892814

* additional tests

* fixed test

* https://github.com/corona-warn-app/cwa-app-android/pull/2232#discussion_r569357630

* https://github.com/corona-warn-app/cwa-app-android/pull/2232#discussion_r569358473

* https://github.com/corona-warn-app/cwa-app-android/pull/2232#discussion_r569557960

* https://github.com/corona-warn-app/cwa-app-android/pull/2232#discussion_r569559428

* https://github.com/corona-warn-app/cwa-app-android/pull/2232#discussion_r569359216



* Test formatting.

* made reading otp from storage failsafe

* fail fast in case gson does not initialize members

* take advantage of compilereturn flow check, no Elvis-Op in unit tests

* better name for test

Co-authored-by: Matthias Urhahn <matthias.urhahn@sap.com>
Co-authored-by: harambasicluka <64483219+harambasicluka@users.noreply.github.com>