Major improvements (added encryption functionality and more)

[binsky08]

• Add support for adding credentials
• Add support for editing credentials
• Add support for deleting credentials
• Add support for adding files
• Add support for downloading files
• Add support for deleting files
• Add app settings
• Add a vault refresh button
• Add a vault lock button
• Add a loading dialog when the vault loads
• Deactivate the computationally expensive OTP generation if no OTP has been specified
• Add support for user CA certificates (requires at least Android API level 24)
• Update to gradle version 6.1.1
• Update openssl to version 1.1.1 stable
• Migrate to AndroidX package names
• Add troubleshooting section to the project README (for developers)
• General improvements
• Solved issues: #19 #25 #72

[binsky08]

This is what i have seen today, didn't get time to check all the changed files though, but there is a few things we can check around here. Not all of my comments require a change on the code, but are more like "things we can discuss".

I have not checked the Java encryption implementation though, as i believe that at the end only one of the two should stay on the final revision. If you think that the java implementation is better we can discuss that though.

Java crypto part completely removed