Fix insufficint contrast on Vision Settings toggles.

Bug: 29161887
Change-Id: I633ed3cf0fa00d3b697377118580471afaf30132
diff --git a/res/values/themes.xml b/res/values/themes.xml
index 0009bed..ffe802e 100644
--- a/res/values/themes.xml
+++ b/res/values/themes.xml
@@ -124,6 +124,7 @@
 
     <style name="SetupWizardAccessibilitySwitchBarTheme" parent="ThemeOverlay.SwitchBar.Settings">
         <item name="switchBarBackgroundColor">@color/material_blue_500</item>
+        <item name="android:colorControlActivated">@android:color/white</item>
     </style>
 
     <!-- Theme with no local references, used by AccountPreferenceBase where we have to inflate