refactor: Change Java to Kotlin within ReactNativeGoogleMobileAds related files

[l2hyunwoo]

Description

• Since react-native@73 using Kotlin is default for android module. Even if it can't bring effect to this project, most of references, documents will only support Kotlin. The conversion to Kotlin within the Android module should be done slowly, to make it easier to maintain the module in the future.

Related issues

Release Summary

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • [x] Yes
• My change supports the following platforms;
  • [x] Android
  • [ ] iOS
• My change includes tests;
  • [ ] e2e tests added or updated in __tests__e2e__
  • [ ] jest tests added or updated in __tests__
• [ ] I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • [ ] Yes
  • [x] No

Test Plan

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[CLAassistant]

All committers have signed the CLA.

[github-actions[bot]]

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[DoctorJohn]

These changes actually look great! They're much easier to review by looking at the relevant commits directly. @dylancom, since I can't see the CI test results, I assume the test workflows have yet to be approved for this PR?

[dylancom]

@mikehardy Could you have a look at this?

[mikehardy]

:tada: This PR is included in version 14.2.4 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: