[ReactNative] Add more markers and fix FPS graph

Summary:
@public

Add marker to show JavaScript download duration + flow arrows to show the origin
of the UI blocks being flushed.
Also fixed the condition on `RCTPerfStats`, UI and JS graphs were being created
at startup time, now they're just created on the first time they're shown.

Test Plan:
The markers:

{F22577660}

To check the FPS graph, enable it on the DevMenu, and it should appear initially
empty, instead of previously filled as before.

React/Modules/RCTUIManager.m

@@ -979,7 +979,9 @@ RCT_EXPORT_METHOD(findSubviewIn:(NSNumber *)reactTag atPoint:(CGPoint)point call
   [_pendingUIBlocksLock unlock];
 
   // Execute the previously queued UI blocks
+  RCTProfileBeginFlowEvent();
   dispatch_async(dispatch_get_main_queue(), ^{
+    RCTProfileEndFlowEvent();
     RCTProfileBeginEvent();
     for (dispatch_block_t block in previousPendingUIBlocks) {
       block();