[ui5-select][a11y] can't use Screen Reader on ui5-select

Bug Description

ui5-select is not usable with a screen reader, it opens the options but you can't focus on the options to actually select them nor nothing, you can also open multiple ui5-selects at the same time this way.

Expected Behavior

I expected the select to be announced as a "menu" and then I would be sent to the selected option, where I could press up or down to navigate the menu to the option I want.

Steps to Reproduce

Open https://sap.github.io/ui5-webcomponents/playground/components/Select/

Log Output / Stack Trace / Screenshots

https://user-images.githubusercontent.com/2848323/129414940-a22f7fde-0e35-441a-8c2f-1ea8c2ef0ca5.mp4

Context

• UI5 Web Components version: the one on the docs
• OS/Platform: macOS
• Browser: Firefox and Chromium
• Affected component: ui5-select

Priority

• Low

• Medium

• High

• Very High

• Accessibility issue - Medium or High

Stakeholder Info (if applicable)

• Organization: CIM