Add fabric on Android

[okwasniewski]

Summary:

• Configure gradle files
• Implement / extend the generated native interfaces
• Ensure backwards compatibility by splitting implementations into two folders.

Used examples / references:

• React native slider implementation

Test Plan:

• Setup fabric example to test the app using RN 0.70 RC

Test plan for RN <0.70:

1. Set newArchEnabled to true inside example/android/gradle.properties
2. Add this line in example/android/app/src/main/jni/Android.mk file.

+ include $(NODE_MODULES_DIR)/@react-native-community/slider/android/build/generated/source/codegen/jni/Android.mk
include $(CLEAR_VARS)

In the same file above, go to the LOCAL_SHARED_LIBRARIES setting and add the following line:

    libreact_codegen_rncore \
+   libreact_codegen_ReactSlider \
    libreact_debug \

3. Update example/android/app/src/main/jni/MainComponentsRegistry.cpp

#include <react/renderer/components/rncore/ComponentDescriptors.h>
+ #include <react/renderer/components/ReactSlider/ComponentDescriptors.h>

...

MainComponentsRegistry::sharedProviderRegistry() {
  auto providerRegistry = CoreComponentsRegistry::sharedProviderRegistry();

  // Custom Fabric Components go here. You can register custom
  // components coming from your App or from 3rd party libraries here.
  //
  // providerRegistry->add(concreteComponentDescriptorProvider<
  //        AocViewerComponentDescriptor>());
+ providerRegistry->add(concreteComponentDescriptorProvider<RNCSliderComponentDescriptor>());

  return providerRegistry;
}

Then, to launch the project run:

npm run install && npm run example-android

[okwasniewski]

This PR is part of #380 - linking this to the issue.

[BartoszKlonowski]

I left some very minor comments, but it's totally well done!
Thank you! 👍

[BartoszKlonowski]

👍