title: readonly property on input does not prevent the keyboard to show
body: <p>I am trying to use a native date picker in Ionic 2 by handling the click event on the input:</p>
  
  <pre><code>&lt;ion-item&gt;
      &lt;ion-label stacked&gt;Start&lt;/ion-label&gt;
      &lt;ion-input type="datetime" readonly="true" placeholder="Choose a date" formControlName="date" (click)="onDateClick($event)" tappable&gt;&lt;/ion-input&gt;
    &lt;/ion-item&gt;
  </code></pre>
  
  <p>But the <code>readonly</code> property does not seem to prevent the keyboard to show one first time, I have to click again in order to get the native date picker to show (handled in <code>onDateClick($event)</code>).</p>
  
  <p>I tried to use the <code>disabled</code> property, which works but the style is different and somehow it looks like it is not a good pattern when using Reactive Forms...</p>
  
  <p>Any idea how to prevent this first show of the keyboard?</p>
  
  <p>***** EDIT *****</p>
  
  <p>Here is the <code>onDateClick</code> function</p>
  
  <pre><code>onDateClick(e: Event) {
      console.log('NewEventPage#onDateClick');
  
      if (this.platform.is('cordova')) {
        this.datePicker.show({
          date: new Date(),
          mode: 'datetime',
          allowOldDates: false
        }).then(
          date =&gt; {
            console.log('Picked date ', date);
            this.startDate = moment(date);
            this.newEventForm.get('date').setValue(moment(date).format('LLLL'));
          },
          err =&gt; console.log('Error occurred while getting date: ', err)
        );
      } else {
        let tmp = new Date();
        this.startDate = moment(tmp);
        this.newEventForm.get('date').setValue(moment(tmp).format('LLLL'));
      }
    }
  </code></pre>