Migrate Event class (#49887)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49887

## Changelog:
[Internal] -

As in the title.

This was the last Java file in the `events-common` module, so now it can be a pure Kotlin one.

NOTE: I opted to make the base method `Event.getEventData` public (instead of previously protected), as based on the usage patterns by the users this seems to make the most sense in this context.

Reviewed By: cortinico

Differential Revision: D70777597

fbshipit-source-id: fdaa52e4400ad8e86a8711daf359eb5a10974d05