Add test feature to allow faking high and low risk encounters via exposure windows.

...y/modules/exposurewindow/DefaultExposureWindowProvider.kt

...esters/assets/exposure-windows-increased-risk-random.json

...iceForTesters/assets/exposure-windows-lowrisk-random.json

...y/modules/exposurewindow/DefaultExposureWindowProvider.kt

...arby/modules/exposurewindow/FakeExposureWindowProvider.kt

...app/test/risklevel/ui/TestRiskLevelCalculationFragment.kt

...klevel/ui/TestRiskLevelCalculationFragmentCWAViewModel.kt

...sters/res/layout/fragment_test_risk_level_calculation.xml

...rc/main/java/de/rki/coronawarnapp/storage/TestSettings.kt

...est/java/de/rki/coronawarnapp/storage/TestSettingsTest.kt

...arby/modules/exposurewindow/ExposureWindowProviderTest.kt

...arby/modules/exposurewindow/ExposureWindowProviderTest.kt