Add a modifier for reading percentage in sliders

Slider modified its state description to read the raw value. This
creates a new modifier that can be used for Slider (or anything with a
percentage) to have that read as the state description.

Test: manual, using talkback
Test: atest BrightnessSliderTest DualIconSliderTest
Bug: 422986518
Flag: EXEMPT bugfix
Change-Id: I23ea983f9f644479919e0f6c4dc0a8b6f9317f97