Add checkstyle plugin and config

Looks like the last few commits were still not using the newly added code configuration. To catch such things in the future, I guess makes sense to add the checkstyle plugin as we do in the mailbox.

I added the configuration in a first commit, the CI then fails, as expected because of the violations: https://code.briarproject.org/briar/onionwrapper/-/jobs/28365

After reformatting using AS, the CI succeeds again.

Unfortunately, the plugin does not work on the buildSrc sources, but I think it's certainly better than nothing.

mentioned in merge request !2 (merged)

assigned to @sebkur

requested review from @akwizgran

Looks good, thanks. The changes match what Android Studio's Ctrl+L would do, so hopefully we won't get conflicts.

merged

mentioned in commit 88d61cbe