Minor bug in random generator for selection prop

Summary:
`.sort()` will sort on the string value by default, so if you generate [18, 8], they will stay in that order. Adding comparer to ensure values are sorted numerically.

Found a bug in RNTester.

Ran RNTester and confirmed that bug could be reproduced.

[IOS][BUGFIX][RNTester] - patch test to implement desired behavior.
Closes #16871

Differential Revision: D6371550

Pulled By: shergin

fbshipit-source-id: 84866d1eb02c2be51cd15a60490604d28fa18973

Author: rozele

RNTester/js/TextInputExample.ios.js

@@ -284,7 +284,7 @@ class SelectionExample extends React.Component<$FlowFixMeProps, SelectionExample
   }
 
   selectRandom() {
-    var positions = [this.getRandomPosition(), this.getRandomPosition()].sort();
+    var positions = [this.getRandomPosition(), this.getRandomPosition()].sort((a, b) => a - b);
     this.select(...positions);
   }