Observer is now abstract class.

Reviewers: #material_motion, O6 Material Android platform reviewers, O2 Material Motion, featherless

Reviewed By: #material_motion, O6 Material Android platform reviewers, O2 Material Motion, featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D2176

library/src/main/java/com/google/android/material/motion/streams/MotionObservable.java

@@ -69,7 +69,7 @@ public MotionObservable(Subscriber<MotionObserver<T>> subscriber) {
   /**
    * An observer with an additional {@link #state(int)} method.
    */
-  public static abstract class MotionObserver<T> implements Observer<T> {
+  public static abstract class MotionObserver<T> extends Observer<T> {
 
     @Override
     public abstract void next(T value);