TM iOS: Run RCTPlatform methods on the main queue
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

myntra/react-native,arthuralee/react-native,myntra/react-native,arthuralee/react-native,pandiaraj44/react-native,javache/react-native,facebook/react-native,hammerandchisel/react-native,hoangpham95/react-native,exponent/react-native,exponentjs/react-native,exponent/react-native,facebook/react-native,facebook/react-native,hammerandchisel/react-native,myntra/react-native,hammerandchisel/react-native,javache/react-native,javache/react-native,javache/react-native,exponent/react-native,pandiaraj44/react-native,javache/react-native,myntra/react-native,pandiaraj44/react-native,pandiaraj44/react-native,hoangpham95/react-native,pandiaraj44/react-native,myntra/react-native,arthuralee/react-native,facebook/react-native,arthuralee/react-native,hoangpham95/react-native,javache/react-native,hoangpham95/react-native,hoangpham95/react-native,myntra/react-native,exponent/react-native,pandiaraj44/react-native,hammerandchisel/react-native,janicduplessis/react-native,janicduplessis/react-native,pandiaraj44/react-native,javache/react-native,hammerandchisel/react-native,myntra/react-native,exponent/react-native,janicduplessis/react-native,janicduplessis/react-native,facebook/react-native,janicduplessis/react-native,exponent/react-native,janicduplessis/react-native,exponent/react-native,exponentjs/react-native,pandiaraj44/react-native,arthuralee/react-native,javache/react-native,hammerandchisel/react-native,facebook/react-native,hammerandchisel/react-native,facebook/react-native,janicduplessis/react-native,hoangpham95/react-native,facebook/react-native,exponentjs/react-native,exponentjs/react-native,javache/react-native,exponentjs/react-native,hoangpham95/react-native,exponentjs/react-native,hammerandchisel/react-native,janicduplessis/react-native,exponent/react-native,facebook/react-native,hoangpham95/react-native,myntra/react-native,myntra/react-native,exponentjs/react-native,exponentjs/react-native