[Android] Player is accessed on the wrong thread

## 🐛 Bug Report

When the video is playing and seek to some position (using SeekTo), it shows in output window this error:

```
10-25 16:29:22.618 W/SimpleExoPlayer( 4743): Player is accessed on the wrong thread. See https://google.github.io/ExoPlayer/faqs.html#what-do-player-is-accessed-on-the-wrong-thread-warnings-mean
10-25 16:29:22.618 W/SimpleExoPlayer( 4743): java.lang.IllegalStateException
10-25 16:29:22.618 W/SimpleExoPlayer( 4743): at com.google.android.exoplayer2.SimpleExoPlayer.verifyApplicationThread(SimpleExoPlayer.java:1192)
10-25 16:29:22.618 W/SimpleExoPlayer( 4743): at com.google.android.exoplayer2.SimpleExoPlayer.getPlaybackParameters(SimpleExoPlayer.java:946)
```

Note that the video still working and seek to the position, but show this error.

### Expected behavior

The video seek to the position and don't show any error message.

### Reproduction steps

Play some video and when it update the progress in PositionChanged, call MediaManager.SeekTo(position).

### Configuration

**Version:** 0.8.18

**Platform:**
- [ ] :iphone: iOS
- [x ] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms

Trace the Android path from PositionChanged to MediaManager.SeekTo and compare it with the SimpleExoPlayer.verifyApplicationThread warning shown in the report. Reproduce playback and seeking on version 0.8.18; done means seeking still works without the wrong-thread error.

Written by the indexing model from the issue text.