Use Android SDK 27 in React Native

Summary:
Upgrade React Native to Android SDK 27 again, following the reversal in D9886607 (https://github.com/facebook/react-native/commit/68c7999c25efbc6fabf67e4130ac086c401b88e0).

The SDK 27 is actually available internally in an alternate location that is suitable for use cases like React Native's. For future reference, SDK 28 is also available for use in this location.

Reviewed By: axe-fb

Differential Revision: D9929066

fbshipit-source-id: 9413f891d5587293a30544351340e9407a2dce55