Add Flow types for Platform.select [5/5]

Summary: This change adds type information for `Platform.select` to make sure things are properly typed. This is the last diff in a stack that actually enables the newly fixed/added annotations and it will only land once all the type errors (320+) are fixed.

Reviewed By: sahrens

Differential Revision: D13318805

fbshipit-source-id: af0475d1a2f1e5ace4d513be48827e3659bd7f62

Libraries/Utilities/PlatformOS.android.js

@@ -5,7 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  *
  * @format
- * @flow
+ * @flow strict
  */
 
 'use strict';