Add a way to access the underlying JavaScriptCore context

Summary:
**Motivation**

I'm working on a project that uses React Native and needs to add direct synchronous bindings to native stuff through the JavaScriptCore C API. This is because it's performance-sensitive and would benefit from the quickest JS->C path. It does this using cross-platform C++ code that works on both iOS and Android. Most of the infrastructure for getting access to the JSC context is already in React Native actually, just had to add a few more things.

(lexs you mentioned to tag you in this pull request)

**Test plan**

Modify the JavaScriptCore context through the `JSContextRef` returned (eg. add an object at global scope) and verify that it exists in JavaScript.
Closes https://github.com/facebook/react-native/pull/10399

Differential Revision: D4080945

Pulled By: lexs

fbshipit-source-id: 6659b7a01e09fd84475adde183c1d3aca2d4cf09

ReactAndroid/src/main/java/com/facebook/react/bridge/CatalystInstance.java

@@ -75,4 +75,9 @@ public interface CatalystInstance extends MemoryPressureListener {
 
   @VisibleForTesting
   void setGlobalVariable(String propName, String jsonValue);
+
+  /**
+   * Get the C pointer (as a long) to the JavaScriptCore context associated with this instance.
+   */
+  long getJavaScriptContext();
 }

ReactAndroid/src/main/java/com/facebook/react/bridge/ReactContext.java

@@ -339,4 +339,11 @@ public class ReactContext extends ContextWrapper {
     }
     return mCurrentActivity.get();
   }
+
+  /**
+   * Get the C pointer (as a long) to the JavaScriptCore context associated with this instance.
+   */
+  public long getJavaScriptContext() {
+    return mCatalystInstance.getJavaScriptContext();
+  }
 }

ReactAndroid/src/main/java/com/facebook/react/cxxbridge/CatalystInstanceImpl.java

@@ -347,6 +347,9 @@ public class CatalystInstanceImpl implements CatalystInstance {
   @Override
   public native void setGlobalVariable(String propName, String jsonValue);
 
+  @Override
+  public native long getJavaScriptContext();
+
   // TODO mhorowitz: add mDestroyed checks to the next three methods
 
   @Override