Add GDNative libraries to Android custom Gradle builds

[madmiraal]

Currently, GDNative libraries are not being copied into the custom build project directories when doing an Android Custom Build. The files are simply ignored:

godot/platform/android/export/gradle_export_util.h

Lines 103 to 105 in c8444c3

	Error ignore_so_file(void *p_userdata, const SharedObject &p_so) {
	return OK;
	}

This PR ensures the GDNative libraries are copied to the correct directory; so that they're automatically included when Gradle builds the project.

Fixes #49878

[m4gr3d]

The logic looks straightforward! I'm wondering if we need to delete the added so files on rebuild. Otherwise it seems like a previously added so file might continue to be included in the generated binary even if it's removed from the project.

[madmiraal]

I'm wondering if we need to delete the added so files on rebuild.

I thought about that. The problem is, it's impossible to tell what library has been removed, and we don't want to simply remove all the libraries, because the user may, as part of their custom build, be including other libraries manually.

[pouleyKetchoupp]

I'm wondering if we need to delete the added so files on rebuild.

I thought about that. The problem is, it's impossible to tell what library has been removed, and we don't want to simply remove all the libraries, because the user may, as part of their custom build, be including other libraries manually.

We could include a .json file (or equivalent) in the export folder that lists the GDNative library files that have been included, so on rebuild the process can know which files to remove.

[madmiraal]

Updated to use a JSON to store copied .so files, which are removed at the start of a new build.

[akien-mga]

Thanks!