Fix #2532522: Slider ARIA attribute instantiation

Solves the bug in this ticket, where `aria-valuenow` and `aria-valuetext` aren't properly rendered upon instantiation.

Ping @lsmith and @jconniff because I've never worked with Slider before, but I think moving the ARIA attribute setting to `_setPosition` is the correct way to go with fixing the bug, since that's what being called in `_syncThumbPosition`, which is the override to `syncUI`.