MediaSession2: Add reset() to the MediaPlayerBase

This would be used in response to
  - MediaController2#play()
  - KEYCODE_MEDIA_STOP

Bug: 64098437
Test: passed MediaComponentTest
Change-Id: I7cd4d855d5e1bf8e4260a9b920f30dfe2eea44e1

media/java/android/media/MediaPlayerBase.java

@@ -113,6 +113,11 @@ public abstract class MediaPlayerBase implements AutoCloseable {
      */
     public abstract void pause();
 
+    /**
+     * Resets the MediaPlayerBase to its uninitialized state.
+     */
+    public abstract void reset();
+
     /**
      *
      */