camera3: Make blackLevelPattern optional.

Change-Id: Ic7b115e9f6361e158e7af13442bb781bbf351cbf
diff --git a/core/java/android/hardware/camera2/CameraCharacteristics.java b/core/java/android/hardware/camera2/CameraCharacteristics.java
index 83314ea..7f61558 100644
--- a/core/java/android/hardware/camera2/CameraCharacteristics.java
+++ b/core/java/android/hardware/camera2/CameraCharacteristics.java
@@ -620,6 +620,7 @@
      * channels in the camera sensor.</p>
      * <p>The values are given in row-column scan order, with the first value
      * corresponding to the element of the CFA in row=0, column=0.</p>
+     * <p><b>Optional</b> - This value may be {@code null} on some devices.</p>
      */
     public static final Key<int[]> SENSOR_BLACK_LEVEL_PATTERN =
             new Key<int[]>("android.sensor.blackLevelPattern", int[].class);