title: Change playback speed in exoplayer
body: <p>I am trying to add a button from which I can slow down the playback speed. The code looks like this:</p>
  <pre><code>fun setupButtons(view: View){
          buttonContainer = view.findViewById(R.id.button_container)
          slowPlayButton = view.findViewById(R.id.slow_button)
          slowPlayButton.setOnClickListener({
              if (!isSlowPlaying) {
                  player.setPlaybackSpeed(0.03f)
              } else {
                  player.setPlaybackSpeed(1.0f)
              }
              isSlowPlaying = !isSlowPlaying
          })
      }
  </code></pre>
  <p>I have verified with debugger that <code>player.setPlaybackSpeed(0.03f)</code> is called, but the speed of the video is not changed.</p>
  <p>If I instead set the playback speed when I set up the player, it is playing slowly.</p>
  <pre><code>private fun createAndShowVideo(baseFile: String) {
          val videoFile = baseFile + &quot;.mp4&quot;
          val mediaItem: MediaItem = MediaItem.fromUri(videoFile.toUri())
          player.setMediaItem(mediaItem);
          player.prepare();
          player.addListener(this)
          player.setPlaybackSpeed(0.03f)
      }
  </code></pre>
  <p>What do I need to do to make the button call take effect?</p>