Fix warnings in SentryFlutter.kt and SentryFlutterPlugin.kt

[denrase]

#skip-changelog

📜 Description

• Fix warnings in SentryFlutter.kt and SentryFlutterPlugin.kt
• Bump Kotlin to 1.8 for example app

💡 Motivation and Context

Closes #3197

💚 How did you test it?

Checked linter in Android Studio

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

[github-actions]

🚨 Detected changes in high risk code 🚨

High-risk code has higher potential to break the SDK and may be hard to test. To prevent severe bugs, apply the rollout process for releasing such changes and be extra careful when changing and reviewing these files:

• packages/flutter/android/src/main/kotlin/io/sentry/flutter/SentryFlutterPlugin.kt

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.71%. Comparing base (8541716) to head (4de2196).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3274      +/-   ##
==========================================
+ Coverage   87.84%   89.71%   +1.87%     
==========================================
  Files         290       95     -195     
  Lines       10003     3393    -6610     
==========================================
- Hits         8787     3044    -5743     
+ Misses       1216      349     -867     

☔ View full report in Codecov by Sentry.
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[denrase]

@stefanosiano Review pls :)