Update docs for DisplayMetrics pixel size fields

Update docs to reflect that values in fields store sizes available for
application (not full screen size).

Bug: 27449789
Change-Id: I92f7552a92fa1b66c5eba4cf28693528cbff21d3

core/java/android/util/DisplayMetrics.java

@@ -154,11 +154,11 @@ public class DisplayMetrics {
     public static final int DENSITY_DEVICE_STABLE = getDeviceDensity();
 
     /**
-     * The absolute width of the display in pixels.
+     * The absolute width of the available display size in pixels.
      */
     public int widthPixels;
     /**
-     * The absolute height of the display in pixels.
+     * The absolute height of the available display size in pixels.
      */
     public int heightPixels;
     /**