Finish migrating SettingsFragment to ViewModel

This also splits up the UI into sub-screens.

device-2021-02-01-085720

Test instructions:

• check that the settings main screen shows icons that are colored like the text in light and dark theme
• enter all new settings screen, check that all settings are enabled as expected (e.g. app lock only enabled when phone has a screen lock)
• change all settings and check that their new value gets persisted across screen rotations as well as leaving settings and coming back
• check that a release build doesn't show developer options and profile pic setting

Closes #1826 (closed), #1942 (closed)