Summary:
**This is the first part of `React.createClass` -> ES2015 classes migration.
1. Rewritten UI Explorer ActivityIndicator example to ES2015 classes
2. Removed TimerMixin from example.
Motivation:
- ES2015 classes do not support mixins so due to the classes / pure functions as "best practices" it would be better to avoid mixins in examples.
- TimerMixin is covered later and is out of scope of current example.
Closes https://github.com/facebook/react-native/pull/8342
Differential Revision: D3659349
fbshipit-source-id: e1c6f1a3091d60c589303fe6da55b8d132adedc3
Artyom Trityak
5617d41327
