Remove TimerMixin on ProgressBarAndroidExample.android.js (#21501)

Summary:
Related to #21485.
Removed TimerMixin from the `RNTester/js/ProgressBarAndroidExample.android.js` since it is currently not used.

- [x] npm run prettier
- [x] npm run flow-check-ios
- [x] npm run flow-check-android

In progress 🙇

[GENERAL] [ENHANCEMENT] [RNTester/js/ProgressBarAndroidExample.android.js] - remove TimerMixin dependency
Pull Request resolved: https://github.com/facebook/react-native/pull/21501

Reviewed By: TheSavior

Differential Revision: D10218375

Pulled By: RSNara

fbshipit-source-id: c4c12f65855452bc2485f034a0560afd204512f4

RNTester/js/ProgressBarAndroidExample.android.js

@@ -16,11 +16,9 @@ var createReactClass = require('create-react-class');
 var RNTesterBlock = require('RNTesterBlock');
 var RNTesterPage = require('RNTesterPage');
 
-var TimerMixin = require('react-timer-mixin');
-
 var MovingBar = createReactClass({
   displayName: 'MovingBar',
-  mixins: [TimerMixin],
+  _intervalID: (null: ?IntervalID),
 
   getInitialState: function() {
     return {
@@ -29,12 +27,18 @@ var MovingBar = createReactClass({
   },
 
   componentDidMount: function() {
-    this.setInterval(() => {
+    this._intervalID = setInterval(() => {
       var progress = (this.state.progress + 0.02) % 1;
       this.setState({progress: progress});
     }, 50);
   },
 
+  componentWillUnmount: function() {
+    if (this._intervalID != null) {
+      clearInterval(this._intervalID);
+    }
+  },
+
   render: function() {
     return <ProgressBar progress={this.state.progress} {...this.props} />;
   },