refactor(android): add mimeTypes property to ImagePicker class

[xuhcc]

Following the discussion in #301

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?

On Android, even when media type is set to ImagePickerMediaType.Any, it is not possible to select any file except image or video, everything else is greyed out.

What is the new behavior?

Behavior remains the same, but the new code structure would allow customization of picker behavior in subclasses, for example:

class FilePicker extends ImagePicker {

    get mimeTypes() {
        let mimeTypes = Array.create(java.lang.String, 1);
        mimeTypes[0] = '*/*';
        return mimeTypes;
    }
}

[tgpetrov]

@xuhcc Can you rebase this one as well?

[xuhcc]

Yes, rebased.

[xuhcc]

Thanks!