Merge "Camera: Clarify doc for registerAvailabilityCallback" into tm-dev

     * except that it uses {@link java.util.concurrent.Executor} as an argument

     * instead of {@link android.os.Handler}.</p>

     *

     * <p>Note: If the order between some availability callbacks matters, the implementation of the

     * executor should handle those callbacks in the same thread to maintain the callbacks' order.

     * Some examples are:</p>

     *

     * <ul>

     *

     * <li>{@link AvailabilityCallback#onCameraAvailable} and

     * {@link AvailabilityCallback#onCameraUnavailable} of the same camera ID.</li>

     *

     * <li>{@link AvailabilityCallback#onCameraAvailable} or

     * {@link AvailabilityCallback#onCameraUnavailable} of a logical multi-camera, and {@link

     * AvailabilityCallback#onPhysicalCameraUnavailable} or

     * {@link AvailabilityCallback#onPhysicalCameraAvailable} of its physical

     * cameras.</li>

     *

     * </ul>

     *

     * @param executor The executor which will be used to invoke the callback.

     * @param callback the new callback to send camera availability notices to

     *