Fix colors of thick brightness slider

More pleasant colors for the background and icon of the slider. Also,
have the icon change color as the slider passes by.

There doesn't seem to be an easy way to use a vector drawable as an item
while at the same time changing the tint and alpha.

Test: manual
Fixes: 17476414

Change-Id: Ie545c3b681885ea8372808aae8804f58c85f2f67