Fix the dark theme obscures the center marker of audio balance slider.

Root Cause:
The color of center marker is wrong.

Next:
The color of center marker is white when dark theme is enabled; Otherwise, it's black.

Bug: 159028208
Test: manual test & make RunSettingsRoboTests ROBOTEST_FILTER=BalanceSeekBarPreferenceTest & make RunSettingsRoboTests ROBOTEST_FILTER=BalanceSeekBarTest
Change-Id: Id2d548778d1275709448f4c009a8469216950d8c
Merged-In: Id2d548778d1275709448f4c009a8469216950d8c
(cherry picked from commit 34a87388)