[Expressive] Update slider content description

The content description for SliderPreference is now set on
holder.itemView instead of just the internal slider widget. When the
slider content description is updated, notifyChanged is called to ensure
the view is re-bound with the new description.

Bug: 436524151
Test: manual test with Talkback
Flag: EXEMPT bugfix for accessibility
Change-Id: I5a2a6dfb9e8f5d8f3f3504a9372c8f9e25261bb9