Simplify HashUtil method when possible

[Arthur-Milchior]

Now that HashUtil methods are not used in Java anymore (at least once #12301 is merged. But my two commits can be reviewed independently of #12301), I can make them return Kotlin type instead of java type. This ensure better typing as far as nullability goes.

Working on them, I also cleaned every usage of those methods that can be replaced by map/associate/filter.

I believe this PR should be rebased

[david-allison]

Needs a rebase to remove the Sound changes

[lukstbit]

@Arthur-Milchior Build failure:

e: /Users/runner/work/Anki-Android/Anki-Android/AnkiDroid/src/main/java/com/ichi2/libanki/sched/Sched.kt: (162, 20): Unresolved reference: HashUtil

[github-actions[bot]]

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