TM iOS: Run RCTPlatform methods on the main queue

Summary:
This module expects to compute main queue stuffs inside getConstants(), mark it so. This address crashes when running tests:

```
Main Thread Checker: UI API called on a background thread: -[UIApplication keyWindow]
PID: 88318, TID: 22602153, Thread name: com.facebook.react.JavaScript, Queue name: com.apple.root.default-qos.overcommit, QoS: 0
Backtrace:
4   RNTester                            0x0000000100beece8 RCTKeyWindow + 72
5   RNTester                            0x0000000100beeee4 RCTForceTouchAvailable + 148
6   RNTester                            0x0000000100c0b54d -[RCTPlatform getConstants] + 109
```

Sample CI failure: https://circleci.com/gh/facebook/react-native/101079

Reviewed By: PeteTheHeat

Differential Revision: D16198720

fbshipit-source-id: 272eaccf5027d0bf5b2838ed9623ae079fac148e

React/CoreModules/RCTPlatform.mm

@@ -44,6 +44,11 @@ RCT_EXPORT_MODULE(PlatformConstants)
   return YES;
 }
 
+- (dispatch_queue_t)methodQueue
+{
+  return dispatch_get_main_queue();
+}
+
 // TODO: Use the generated struct return type.
 - (NSDictionary<NSString *, id> *)constantsToExport
 {