fix(native-ad): align paid event currency payload type

[sanjaysargam]

Description

NativeAd.addAdEventListener(NativeAdEventType.PAID, ...) exposes a payload type
with currencyCode, but both native implementations emit the JavaScript payload
key as currency.

TypeScript accepts payload.currencyCode, but that property is undefined at
runtime. The actual value is available at payload.currency.

Related issues

Fixes #877

Release Summary

Checklist

• I read the Contributor Guide
  and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • 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
  • No

Test Plan

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[CLAassistant]

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