Picker-dropdown remains on a new page

### Problem description

When a picker is expanded and no interaction is performed until a new page is opened, the picker-dropdown still appears on the new page.

### Expected behavior

The picker-dropdown must not be displayed in other pages.

### Environment

- Tabris.js version: 3.0.0-rc1-dev.20190409
- Device: iPhone 6, Google Pixel
- OS: iOS 12.2, Android 9

### Code snippet

```javascript
const { NavigationView, Page, Picker, contentView } = require('tabris');

const AIRPORTS = [
{
id: 'SFO',
name: 'San Francisco'
},
{
id: 'TXL',
name: 'Berlin Tegel'
},
{
id: 'FRA',
name: 'Frankfurt'
}
];

let navigationView = new NavigationView({
left: 0, top: 0, right: 0, bottom: 0
}).appendTo(contentView);

let page = new Page({
title: 'Page'
}).appendTo(navigationView);

page.append(
new Picker({
left: 20, top: 20, right: 20,
itemCount: AIRPORTS.length,
itemText: (index) => AIRPORTS[index].name,
selectionIndex: 1
})
);

setTimeout(() => {
new Page({
title: 'New Screen',
background: 'green'
}).appendTo(navigationView);
}, 5000);
```
## iOS

## Android