We decided on calling the state file the "vault" a while back. This patch makes
the naming consistent across the codebase. I left "DatabaseImporter" classes
alone, because I'm not sure what a better name for those would be.
This also fixes an issue with the use of the Iconics library where it was
initialized twice. I also removed the dependency to [AndroidX Preference
eXtended](https://github.com/takisoft/preferencex-android), as there don't seem
to be any issues with using the vanilla AndroidX preference library anymore.
This adds an option to automatically lock the app when:
* The back button is pressed
* The device is locked
It's the first step towards implementing #7
