Adding fix for scrim semantics

[pranjal-glowingstar]

Problem :-

On expanding the stacked snackbars, the scrim comes into focus. Talkback announces it as "Unlabeled - Double Tap to Activate". We need to update the string announced.

Root cause :-

There is no semantic content description present in Scrim in order for talkback to recite, hence reciting "Unlabeled".

Fix :-

Added the string resource and created a descriptive string for Scrim.

Validations:-

Build successful and behaviour is working as expected.

(how the change was tested, including both manual and automated tests)

Screenshots

Before	After
Screenshot or description before this change	Screenshot or description with this change

Pull request checklist

This PR has considered:

• Light and Dark appearances
• Automated Tests
• Documentation and demo app examples
• VoiceOver and Keyboard Accessibility
• Internationalization and RTL layouts
• Size classes and window sizes (notched devices, multitasking, different window sizes, etc)

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