Updates Display Size & Text preview background color.

Per guidance from Setup Wizard UX leads.
This matches the colors used by other accessibility feature preview
backgrounds.

Fix: 294443760
Test: Settings > Accessibility > Display size and text
      Observe white background in light mode, black background
      in dark mode.
Change-Id: I904415f91ce69e21477e761876944661ef69cb99

res/drawable/accessibility_text_reading_preview.xml

@@ -20,5 +20,5 @@
     android:shape="rectangle">
 
     <corners android:radius="28dp" />
-    <solid android:color="?androidprv:attr/materialColorSurfaceContainer" />
+    <solid android:color="@color/accessibility_text_reading_background" />
 </shape>

res/values-night/colors.xml

@@ -69,6 +69,8 @@
     <color name="udfps_enroll_progress_help">#607DA7F1</color>
     <color name="udfps_enroll_progress_help_with_talkback">#FFEE675C</color>
 
+    <!-- Accessibility colors -->
+    <color name="accessibility_text_reading_background">@android:color/black</color>
     <!-- Flash notifications colors -->
     <!-- Screen flash notification color selected stroke in color selection dialog -->
     <color name="screen_flash_color_button_outer_circle_stroke_color">#FFFFFF</color>

res/values/colors.xml

@@ -137,6 +137,8 @@
     <color name="SIM_dark_mode_color_pink">#ffff8bcb</color> <!-- Material Pink 300 -->
     <color name="SIM_dark_mode_color_orange">#fffcad70</color> <!-- Material Orange 300 -->
 
+    <!-- Accessibility colors -->
+    <color name="accessibility_text_reading_background">@android:color/white</color>
     <!-- Accessibility Settings icon background colors -->
     <color name="accessibility_feature_background">#5F6368</color> <!-- Google grey 700 -->
     <color name="accessibility_magnification_background">#F50D60</color>