Handle seek requests in AudioService

AudioService handles seek requests to RemoteControlClients.
Discard old or unprocessed seek requests when handling a new one.
Update javadoc for OnPlaybackPositionUpdateListener interface.

bug 8120740

Change-Id: Id17c2852b24d4b8af85e472a177484b98cfeba05

media/java/android/media/RemoteControlClient.java

@@ -683,9 +683,12 @@ public class RemoteControlClient
         /**
          * Called on the implementer to notify it that the playback head should be set at the given
          * position. If the position can be changed from its current value, the implementor of
-         * the interface should also update the playback position using
+         * the interface must also update the playback position using
          * {@link RemoteControlClient#setPlaybackState(int, long, int)} to reflect the actual new
          * position being used, regardless of whether it differs from the requested position.
+         * Failure to do so would cause the system to not know the new actual playback position,
+         * and user interface components would fail to show the user where playback resumed after
+         * the position was updated.
          * @param newPositionMs the new requested position in the current media, expressed in ms.
          */
         void onPlaybackPositionUpdate(long newPositionMs);