Rename Gradle modules

[oguzkocer]

This PR renames the native/android folder as native/kotlin which is now also consistent with native/swift.

It also renames the wp_api Gradle module as api:kotlin, and the wp_api_kotlin module as api:android. When these modules are used by consumers, they'll have the rs.wordpress prefix, so the wp prefix wasn't useful. Instead of using api_kotlin & api_android, the submodule structure is preferred in Gradle. (and something we also use in other projects)

Note that there is a good chance I broke something with these changes, but we are still in the early phases of the Kotlin wrapper, so we should be able to quickly address any issues that might arise.

[crazytonyli]

In #77, I have used WordPressAPIInternal as the name of wp_api's Swift API. If I understand correctly, that's the equivlant of rs.wordpress.api.kotlin package. We probably don't need to use a similar name, but just though it's worth mentioning anyways.

I included "internal" in the module name because app developers should not import this "internal" module, even though they technically can. Naming the module as WordPressAPIInternal should be an obvious indicator that the module is designed to be used internally in WordPressAPI (a.k.a the wordpress-rs Swift package).

[oguzkocer]

rs.wordpress.api.kotlin is a proper JVM library and is planned to be used externally. Any non-Android JVM client/library should use this package over rs.wordpress.api.android which is a convenience wrapper for Android clients/libraries.

[wzieba]

Looks good - I find the new naming easy to understand and straightforward 👍