[DRAFT] Add support for the Android XR OpenXR vendor

[m4gr3d]

This PR adds support for the Android XR OpenXR vendor, which will provide the ability to run Godot XR apps on Android XR devices via the vendor plugin.

• This is a draft PR as the Android XR emulator doesn't support OpenXR apps, and no Android XR devices is yet available for testing, so no testing / validation is feasible at the moment. The PR will remain in this state until we're able to test and validate the functionality.
• While Android XR uses the Khronos loader, the platform introduces its own set of permissions and manifest entries which prevents us from using the Khronos vendor logic, and requires us to create and use a separate Android vendor.
• This PR limits itself to the OpenXR extensions already supported in Godot core and the vendors plugin. Android XR introduces a new set of OpenXR extensions which we may add support for in future PRs.

[dsnopek]

Do we really need another AAR built? We've never used the FLAVOR define in the C++ anywhere - could we just use the Khronos AAR for Android too?

(The same goes for MagicLeap - I think we could reuse the Khronos AAR for that as well)

[dsnopek]

Ah, I guess it's because the Java code is different.

Well, after PR #227 is merged, this won't be as big of a deal because with that one we do only one C++ build, and then each of the Java builds just reuses the artifacts of that :-)

[m4gr3d]

Yes the new flavor is primarily for the difference in the java code and the manifest. The C++ code and configuration is the same as the others.

[dsnopek]

Ah, I guess you noted it above (I didn't read the description too carefully :-)) but this is the most disappointing thing:

I can't wait until we can test Android XR, even in an emulator!