[Slider] Add current value to talkback for Slider

[asdfff711]

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Talkback currently for Slider, when selected goes "[Seekbar] [Use volume keys to adjust]"

The additional of the line adds:

"[Value is (value)] [Seekbar] [Use volume keys to adjust]"

• [x] Identify the component the PR relates to in brackets in the title. [Buttons] Updated documentation
• [ ] Link to GitHub issues it solves. closes #1234
• [x] Sign the CLA bot. You can do this once the pull request is opened.

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