Restored Hover Effect to Brightness Slider

Initially introduced in b/283799663 it was reverted as a fix for
b/317732024 because making it clickable affects a11y.
Hover state is now manually induced by capturing hover events and
setting hovered to true.

Flag: NA
Test: Manual Test added to go/sysui-pkt-manual-testing
Fixes: 331174325
Change-Id: I7d067d7cad1f013508c86143032e0a1da96b87eb