[Java.Interop] Replace Gendarme with Roslyn FxCop Analyzers.

[jpobst]

Today we use Gendarme for static code analysis on Java.Interop.dll. Unfortunately Gendarme is dated and additionally relies on .mdb files for analysis. This requires compiling with mcs which is deprecated and does not support modern C# features like NRT.

This PR replaces Gendarme with Roslyn FxCop Analyzers, which get around the above issues, as well as provide a few additional benefits:

• They run on every compile so they will be seen as part of the normal development workflow instead of a separate step.
• They do source code analysis instead of binary analysis which allows them to catch more issues.

This PR aims for parity with Gendarme so it configures every analyzer equivalent to Gendarme as severity error (by default these only cause warnings).

If we want to change the severity of a rule, that should be changed in the .editorconfig file.

If we want to ignore an instance of a violation, that goes in the suppression file. VS has a tooltip of "Suppress this error in suppression file" to do it for you, other editors may need to do it by hand, but the syntax is pretty similar to the Gendarme syntax.

[jonpryor]

What is the "nature" of this file? What causes errors to be generated? Any build, meaning we'd see future failures in PR builds? Or just builds within "some supported IDE"?

When an error is found, how is it reported?

What's the workflow?

[jpobst]

These are understood by msbuild and are passed to csc.exe which actually does the source code analysis at compile time (this is source code analysis, not binary analysis like Gendarme or FxCop):

csc.exe ... /analyzerconfig:C:\code\xamarin-android\external\Java.Interop\.editorconfig

They are reported by the compiler just like any other compiler generated warning or error, so this will now fail builds from the msbuild command line, in IDEs, and in CI/PR builds without any additional steps.

Example:
https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=3261455&view=results

If we want to ignore an instance, that goes in the suppression file. VS has a tooltip of "Suppress this error in suppression file" to do it for you, other editors may need to do it by hand, but the syntax is pretty similar to the Gendarme syntax.