title: Keydown event not working in Android Device
body: <p>I write a function using keydown event to let an input only allow number, '-', and BackSpace key. When I test in ionic serve or iOS, it worked. But when I test in Android device, its not working, input field will allow all the character inputed.
  When I use console.log (event), it always returns the key=“Unidentified”, keyCode = 229.</p>
  
  <p>Environment: Ionic 3, Node v6.11, NPM 3.10, Android 5.1.1</p>
  
  <p>I don't know how to fix this.</p>
  
  <p>.ts file</p>
  
  <pre><code>import {Component} from '@angular/core';
  import {NavController} from 'ionic-angular';
  
  @Component({
    selector: 'page-home',
    templateUrl: 'home.html'
  })
  export class HomePage {
    constructor(public navCtrl: NavController) {
  
    }
  
    isValidNumber(event) {
      //console.log(event);
      return /\d|-|Backspace/.test(event.key);
  
    }
  }
  </code></pre>
  
  <p>.html file</p>
  
  <pre><code>&lt;ion-header&gt;
    &lt;ion-navbar&gt;
      &lt;ion-title&gt;Home&lt;/ion-title&gt;
    &lt;/ion-navbar&gt;
  &lt;/ion-header&gt;
  &lt;ion-content&gt;
    &lt;ion-item&gt;
        &lt;ion-input  type="text" (keydown)="isValidNumber($event)"&gt;&lt;/ion-input&gt;
    &lt;/ion-item&gt; 
  &lt;/ion-content&gt;
  </code></pre>