Add caption window color attribute

The WebVTT rendering implementation interprets window color as region
color, since there is no window for non-region captions.

BUG: 12020757
Change-Id: Ie891e1343d718783fc32c712860f886de22558ec
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index 0dffc17..ec3819a 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -3741,6 +3741,16 @@
                 "accessibility_captioning_edge_color";
 
         /**
+         * Integer property that specifes the window color for captions as a
+         * packed 32-bit color.
+         *
+         * @see android.graphics.Color#argb
+         * @hide
+         */
+        public static final String ACCESSIBILITY_CAPTIONING_WINDOW_COLOR =
+                "accessibility_captioning_window_color";
+
+        /**
          * String property that specifies the typeface for captions, one of:
          * <ul>
          * <li>DEFAULT