[ReactNative] Add RCTAssertThread and restrict -[UIManager addUIBlock:] to _shadowQueue

Summary:
@public

Add `RCTAssertThread` to `RCTAssert.h` for convenience when checking the current/queue,
it accepts either a `NSString *`, `NSThread *` or `dispatch_queue_t` as the object to be checked

Also add a check to `-[RCTUIManager addUIBlock:]` - There was a discussion on github (https://github.com/facebook/react-native/issues/1365)
due to the weird behavior caused by calling it from a different thread/queue (it might be added after `batchDidComplete` has been called
and will just be dispatched on the next call from JS to objc)

Test Plan:
Change `-[RCTAnimationExperimentalManager methodQueue]` to return `dispatch_get_main_queue()` and run the 2048 example,
it should dispatch with a helpful message (screenshot on the comments)

React/Modules/RCTUIManager.m

@@ -390,6 +390,10 @@ static NSDictionary *RCTViewConfigForModule(Class managerClass, NSString *viewNa
 
 - (void)addUIBlock:(RCTViewManagerUIBlock)block
 {
+  RCTAssertThread(_shadowQueue,
+                  @"-[RCTUIManager addUIBlock:] should only be called from the "
+                  "UIManager's _shadowQueue (it may be accessed via `bridge.uiManager.methodQueue`)");
+
   if (!self.isValid) {
     return;
   }