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
diff --git a/core/java/android/util/DisplayMetrics.java b/core/java/android/util/DisplayMetrics.java
index a747f16..d201ade 100644
--- a/core/java/android/util/DisplayMetrics.java
+++ b/core/java/android/util/DisplayMetrics.java
@@ -154,11 +154,11 @@
     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;
     /**