fix Android CI (#25655)

Summary:
This PR adds dummy **is_androidx** option to **rn_java_library** buck rule, and fix feature parity with FB.

## Changelog

[Android] [Changed] - add is_androidx option to rn_java_library buck rule
Pull Request resolved: https://github.com/facebook/react-native/pull/25655

Test Plan: Android CI is green

Differential Revision: D16259555

Pulled By: cpojer

fbshipit-source-id: 464abffccd9ca7a7c5332ab5add9b4319038fd78

tools/build_defs/oss/rn_defs.bzl

@@ -179,6 +179,7 @@ def rn_plugin_apple_library(**kwargs):
     native.apple_library(**kwargs)
 
 def rn_java_library(*args, **kwargs):
+    is_androidx = kwargs.pop("is_androidx", False)
     native.java_library(*args, **kwargs)
 
 def rn_java_annotation_processor(*args, **kwargs):