Employ coding guidelines

[auermich93]

I thought it would be nice to follow some coding guidelines, primarily in terms of documentation and code structure.
The code base lacks here some points, for instance:

• There is not really any sort of java doc. I would suggest java doc for at least public methods and inline comments for key parts.
• There are a couple of unused methods. Either remove them or declare them as unused.
• There are multiple methods that declare exceptions that are actually not thrown.
• There are classes, e.g. ApkDecoder where class variable/constant definitions are scattered throughout the class.

I would like to hear from you.

[iBotPeaches]

The javadoc I'm taking more seriously after rebooting the maven libraries. I'm noticing that is quite painful without them on public methods, which is leading to taking a look at the methods exposed or not.

Unused methods I'm aware of and probably just need to clean up.

The exception thrown I ran into during javadoc generation for Maven and I started cleaning up slowly in #2530

The variable/constant scatter - I'm waiting to see if a good linter exists in the Java world that perhaps follows closely with AOSP, so we can enforce these and do a big batch organization.

[iBotPeaches]

Took a pass at removing declared, but unused exceptions - f4c2f9b

[auermich93]

Great. How about using https://checkstyle.sourceforge.io/?