Fix Material Slider day-night theme change in the Volume Dialog.

For some reason color picker doesn't resolve color attributes inside of
it for the material slider. This works perfectly for the exact colors
though. Given that the sliders in the Volume Dialog are always enabled,
we can directly use the attributes.

Flag: com.android.systemui.volume_redesign
Bug: 369994956
Test: manual on the phone. Change current mode from dark to light and
back.

Change-Id: Iddb79cfe761ce2b64ffea9b4f3cde776a482e756