@Nonnull annotation for ReactPackage (#23415)

Summary:
Here are some leftovers from nullable annotations for native modules, discovered while developing native module in Kotlin. This will help improve Kotlin developer experience

[Android] [Changed] - Add Nonnull annotations to ReactPackage
Pull Request resolved: https://github.com/facebook/react-native/pull/23415

Differential Revision: D14064607

Pulled By: cpojer

fbshipit-source-id: af2ce1fc1911ee03c54b20a4fc3a6d1aba9267da

ReactAndroid/src/main/java/com/facebook/react/ReactPackage.java

@@ -34,10 +34,12 @@ public interface ReactPackage {
    * @param reactContext react application context that can be used to create modules
    * @return list of native modules to register with the newly created catalyst instance
    */
+  @Nonnull
   List<NativeModule> createNativeModules(@Nonnull ReactApplicationContext reactContext);
 
   /**
    * @return a list of view managers that should be registered with {@link UIManagerModule}
    */
+  @Nonnull
   List<ViewManager> createViewManagers(@Nonnull ReactApplicationContext reactContext);
 }