fix: not exported type

[tgpetrov]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• All existing tests are passing
• Tests for the changes are included

What is the current behavior?

There is a ImagePickerMediaType enum that seems like it can be added to the demo app:

let context = imagepicker.create({ mode: "single", mediaType: imagepicker.ImagePickerMediaType.Image });

However when I run the app, it crashes with Cannot read property 'Image' of undefined

What is the new behavior?

App doesn't crash with the above changes

Related with #285.