fix(analytics): Expose a constant for the API name used for Gen2 defineData

[x] PR title and description conform to Pull Request guidelines.

Issue #, if available:

Description of changes: Gen2 doesn't have a concept of API names when using defineData, so we hardcode a default name to use instead.

We realized that you need to supply an API name to configure okhttp clients, so customers will need to supply this hardcoded default. Exposing this constant will avoid having customers need to copy/paste a string from documentation.

val plugin = AWSApiPlugin.builder()
  .configureClient(GEN2_DATA_API_NAME) { clientBuilder ->
    // ...
  }
  .build()

Also aligned the value of the constant with Swift to be gen2DataApiName.

How did you test these changes? (Please add a line here how the changes were tested)

Documentation update required?

[x] No
[ ] Yes (Please include a PR link for the documentation update)

General Checklist

[x] Added Unit Tests
[ ] Added Integration Tests
[ ] Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

aws-amplify / amplify-android

fix(analytics): Expose a constant for the API name used for Gen2 defineData #2791