clarify that ANDROID_ID is unique for each user in multi-user environment.

        /**

         * A 64-bit number (as a hex string) that is randomly

         * generated on the device's first boot and should remain

         * constant for the lifetime of the device.  (The value may

         * change if a factory reset is performed on the device.)

         * generated when the user first sets up the device and should remain

         * constant for the lifetime of the user's device. The value may

         * change if a factory reset is performed on the device.

         * <p class="note"><strong>Note:</strong> When a device has <a

         * href="{@docRoot}about/versions/android-4.2.html#MultipleUsers">multiple users</a>

         * (available on certain devices running Android 4.2 or higher), each user appears as a

         * completely separate device, so the {@code ANDROID_ID} value is unique to each

         * user.</p>

         */

        public static final String ANDROID_ID = "android_id";