Fixes#2057. Everybody hate this because of its "privacy-oriented" storage lockdowns, but it needs to happen anyway.
- Update to SDK 30
- Enable requestLegacyExternalStorage flag in manifest
- Update test libraries, mostly related to AndroidX tests
- Fixes to AppConfigTest and DbViewerTaskTest for new ShadowLooper behaviour (default=PAUSED) in Robolectric 4.4
- Update gradle plugin to 3.6.4, the minimal version without updating to SDK 29. This is necessary for fixing some classloader issues with BouncyCastle when running tests
- Update to Robolectric 4.3; also added AndroidX test packages as required
- As first step to Robolectric adaptation, removed no longer valid `constants = BuildConfig.class` in tests