ExoPlayer how to show loading first frame during loading of video

I'm having an API serving the video URI and a (preloaded!) first frame URI. How can I initialize the ExoPlayer with the already available first frame?

@Singleton
class MediaPlayer @Inject constructor(private val context: Context) {

    companion object {
        var exoPlayer: SimpleExoPlayer? = null
        var pauseItem = false
    }

    fun initializeExoplayer(uri: Uri): SimpleExoPlayer {
        if (exoPlayer != null) {
            resetPlayer()
        }

        val trackSelector = DefaultTrackSelector()

        trackSelector.setParameters(
            DefaultTrackSelector.ParametersBuilder()
                .setForceLowestBitrate(true)
                .setMaxAudioBitrate(128_000)
        )

        exoPlayer = ExoPlayerFactory.newSimpleInstance(context, trackSelector)

        val mediaSource = createMediaSource(uri)
        val loopingSource = LoopingMediaSource(mediaSource)

        exoPlayer?.prepare(loopingSource)
        exoPlayer?.volume = 1f

        return exoPlayer!!
    }

    fun resetPlayer() {
        exoPlayer?.playWhenReady = false
        exoPlayer?.stop()
        exoPlayer?.release()
        exoPlayer = null
    }

    private fun createMediaSource(uri: Uri): MediaSource {
        val userAgent = Util.getUserAgent(context, "ExoPlayerIntro")
        val dataSourceFactory = DefaultDataSourceFactory(context, userAgent)

        val mediaSource =
            ProgressiveMediaSource.Factory(dataSourceFactory, DefaultExtractorsFactory())
                .createMediaSource(uri)

        return Objects.requireNonNull(mediaSource, "MediaSource cannot be null")
    }
}

I've also asked the Google Team: https://github.com/google/ExoPlayer/issues/8139

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