Remove isValid from RCTInvalidating

Summary:
We only actually use it on RCTBridge and RCTJavaScriptExecutor, so add it to these interfaces explicitly

React/Base/RCTJavaScriptExecutor.h

@@ -29,6 +29,11 @@ typedef void (^RCTJavaScriptCallback)(id json, NSError *error);
  */
 - (void)setUp;
 
+/**
+ * Whether the executor has been invalidated
+ */
+@property (nonatomic, readonly, getter=isValid) BOOL valid;
+
 /**
  * Executes given method with arguments on JS thread and calls the given callback
  * with JSValue and JSContext as a result of the JS module call.