[Spa] Improve RestrictedSwitchPreference semantics

Now Talkback reads like,
- Off
- Unavailable
- Title
- Summary
- Switch
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Bug: 422439682
Flag: EXEMPT new library
Test: manual test with Talkback
Change-Id: I9e0cd4cb0d09054f9aafd817f9c4893c1c79d524