Merge: Clean up to prepare for NitActivity

This PR cleans the way to replace our current use of `NativeActivity` by our own `NitActivity` a direct subclass to `Activity`. The result of this PR that we are less dependent on `NativeActivity` and the `native_app_glue` module.

Notable modifications:

* Use Java `Activity` instead of `NativeActivity` wherever possible.
* Rename, to follow convention, Nit `NativeActivity` to `NativeNativeActivity` and `Activity` to `NativeActivity`. Thus all previous Java code using the Java `NativeActivity`, will now use Java `Activity`. The name `NativeNativeActivity` is weird, but temporary as the class will be removed once we have our `NitActivity`.

Pull-Request: #1128
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>