Low Risk after update from 1.7.0 RC8 (in increased risk state) to 1.8.0 RC2 (EXPOSUREAPP-3988) (#1745)

* check whether there already is a v2 exposure

* check preconditions when selecting task for execution - not when scheduling it

* fixed tests

* fixed detekt

* removed todo that was already addressed by d4rken

* updated tests

* https://github.com/corona-warn-app/cwa-app-android/pull/1745#discussion_r532568615



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