Streamlined background worker (#804)

* New feature: key retrieval on risk level refresh

* refresh the last time diagnosis keys fetched after transaction call

* TimerHelper update added

* Constants tests

* Formatting

* Fix activity screen orientation.
This solution was proposed by open source contributor: ironjan

* Free space threshold change to 15 Mb

* Updated test

* New streamlined background work implementation

* Removed constraints for periodic worker
Removed key retrieval time calculator

* Added background helper and constants tests

Co-authored-by: Alexander Alferov <a.alferov@sap.com>
Co-authored-by: Philipp Woessner <philipp.woessner@sap.com>