[@nativescript/datetimepicker] DateTimePicker resets date if set first after setting time

I'm using the DateTimePicker component in my application and found a potential bug.

• Setting time field first, it auto sets the date field on the current date. Updating the date field works.
• Setting date field first, it sets the date field and leaves time blank. Setting the time field now, RESETS date field to current day.

There is a property called autoPickTime which defaults to false. I guess thats the reason why setting date does not set the time.

It would be nice to have similar property autoPickDate to prevent overwritting the date or at least check for a set date.

从 @nativescript/datetimepicker 组件开始，复现 issue 中描述的两种字段设置顺序。检查现有的 autoPickTime 行为，并验证先设置日期再设置时间时，不会再将选定日期替换为当天日期；应通过适当的回归测试覆盖此行为。