title: ion-select popover interface is not getting hidden on hardware back button action
body: <p>I used the popover interface on <code>&lt;ion-select&gt;</code> to display pop-over. In an Android device, if the user taps on ion-select but didn't select any option and click the hardware back button, it moves to the previous view but popover interface is still visible. Please help.</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;ion-select interface="popover" [(ngModel)]="item.productType" placeholder="Please select" multiple="false" ionChange="onChange($event)" &gt;
      &lt;ion-option *ngFor="let opt of options" [value]="opt.value"&gt;{{opt.label}}&lt;/ion-option&gt;
  &lt;/ion-select&gt;
  </code></pre>