Clean up old Java code

[geekygecko]

There is still some Java code in the project. It would be good to check if it's still needed and convert it over to Kotlin.

• [x] EpisodeManagerTest.java
• [x] FileStorage.java
• [x] FileUtil.java
• [ ] OldDataOpenHelper.java
• [x] PCBottomSheetBehavior.java
• [x] ShiftyCustomAudio.java
• [ ] ShiftyTrimSilenceProcessor.java
• [x] StringUtil.java
• [x] ~There are also proto generated files in this directory but they can be removed in favor of linking proto generation with Java/Kotlin compilation tasks.~ -> This isn't currently feasible. See this comment.

[MiSikora]

There are also proto generated files in this directory but they can be removed in favor of linking proto generation with Java/Kotlin compilation tasks.

From Slack convo.

Since the source of the generated code isn't in this repo it seems a bit tricky to generate it in a gradle task. Particularly for open source contributors since the repo is private.

So this probably won't happen any time soon. I'm marking this check point as "done".

[MiSikora]

Managed to removed generated proto classes in #1831.