Fixes #29 - Migrate Kotlin code to raw Java

[freakboy3742]

Replaces the use of Kotlin with native Java.

Why?

1. Over the last month, we've had 2 failures introduced by incompatibilities in updates to Android Kotlin tooling.
2. We're not relying on the advantageous features of Kotlin as a language to speed our development process.
3. Requiring an additional language increases the barrier to entry for contribution.
4. The fact that our template uses Kotlin requires every Briefcase Android to download more plugins and libraries; switching to Java saves a couple of hundred MB of downloads.
5. Kotlin 1.5 (the most recent version as of Jun 2021) is incompatible with the Kotlin-gradle-plugin, and the migration docs provided by the Kotlin team are... not helpful.

Since we appear to be able to fix some non-trivial headaches by making this change, I see no reason to preserve Kotlin.

PR Checklist:

• [x] All new features have been tested
• [x] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct