Use volatile for sDidInit in ReactBridge

Summary: In D17747958 I added a function to check the value of a static variable, but forgot to make sure it was accessed in a thread-safe manner. Adding `volatile` to the flag

Reviewed By: makovkastar

Differential Revision: D17763888

fbshipit-source-id: f227b69de1a0df6493424da3b276529555999f70

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

@@ -16,7 +16,7 @@ public class ReactBridge {
   private static volatile long sLoadStartTime = 0;
   private static volatile long sLoadEndTime = 0;
 
-  private static boolean sDidInit = false;
+  private static volatile boolean sDidInit = false;
 
   public static boolean isInitialized() {
     return sDidInit;