ui5-select: aria-expanded attribute is missing in initial status

[zhangliangyi]

Describe the bug
When rendering a ui5-select, the aria-expand property is missing initially. Until user opens the option popover, the property appears and works as expected.

To reproduce
Steps to reproduce the behavior:

1. Go to https://sap.github.io/ui5-webcomponents/playground/components/Select/
2. Check the dom in any ui5-select sample and the aria-expanded property is not found.
3. Click the component to open the popover, then aria-expanded property appears as an attribute.

Expected behavior
The aria-expand="false" should be in the DOM initially when a ui5-select is rendered.

Context

• UI5 Web Components version: 0.29.7
• OS/Platform: MacOS
• Browser (if relevant): Edge
• Other information: N/A

Affected components (if known)
ui5-select

Organization: (if applicable)
SAP SF

Priority: (optional) (Low, Medium, High or Very High)
Medium

Setting priority would indicate the importance of the issue
and would help the process of dispatching and processing.

To set the appropriate priority, you need to consider the impact
and if the issue:

• Breaks entire application or system - High and Very High
• Is accessibility one (Screan Reader, Keyboard Handling) - Medium and High
• Is functional one and makes a single component unusable - Medium and High
• Is visual, styling one - Low and Medium

Note: the priority would be re-evaluated by the issue processors, according to the project internal criterias

Business impact: (mandatory for "Very High")