Images are rendered into an ImageView (on top of the video shutter).
The image view is set to the images emitted by ExoPlayer's ImageOutput
and cleared when there is no longer a selected image track.
In order to keep the existing behavior of video tracks to only clear
the old output once the new first frame is rendered (avoiding short
periods of black between playlist items), we have to reorder this code
slightly to make it work for video and images. Both are treated in the
same way. If both are enabled, video takes precedence.
As the UI module only depends on the common module, we can't direcly
add the ImageOutput to ExoPlayer. This is done via reflection if
the provided Player is an ExoPlayer.
#cherrypick
PiperOrigin-RevId: 643320666
