Wire up RuntimeExecutorFlushing to MobileConfig

Summary:
With D27975839, RuntimeExecutor will be able to start flushing the queued up NativeModule calls in every call from C++ -> JavaScript. We're going to test this feature to measure its impact. In this diff, I wire up the the MobileConfig to ReactFeatureFlags.

Changelog: [Internal]

Reviewed By: JoshuaGross

Differential Revision: D27978112

fbshipit-source-id: 47e1e74398c62755bb0fdc6b54b7fd3aa47eb877

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

@@ -551,7 +551,7 @@ public class CatalystInstanceImpl implements CatalystInstance {
 
   @Override
   public RuntimeExecutor getRuntimeExecutor() {
-    return getRuntimeExecutor(false);
+    return getRuntimeExecutor(ReactFeatureFlags.enableRuntimeExecutorFlushing());
   }
 
   public native RuntimeExecutor getRuntimeExecutor(boolean shouldFlush);

ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java

@@ -72,4 +72,24 @@ public class ReactFeatureFlags {
 
   /** Enables MapBuffer Serialization */
   public static boolean mapBufferSerializationEnabled = false;
+
+  /** An interface used to compute flags on demand. */
+  public interface FlagProvider {
+    boolean get();
+  }
+
+  /** Should the RuntimeExecutor call JSIExecutor::flush()? */
+  private static FlagProvider enableRuntimeExecutorFlushingProvider = null;
+
+  public static void setEnableRuntimeExecutorFlushingFlagProvider(FlagProvider provider) {
+    enableRuntimeExecutorFlushingProvider = provider;
+  }
+
+  public static boolean enableRuntimeExecutorFlushing() {
+    if (enableRuntimeExecutorFlushingProvider != null) {
+      return enableRuntimeExecutorFlushingProvider.get();
+    }
+
+    return false;
+  }
 }