Reduce brightness-slider bottom margin to 16dp, as per spec

https://www.figma.com/file/8Q7oe3QpvXNGpg2k61YvAt/Shade?node-id=5508%3A162185

Bug: 203537805
Test: Manually, shared updated screenshot with designer
Change-Id: Ibf05654cd9fd494f74a4661768d5ec5585e98a3d
1 file changed