FakeMediaSource and FakeTimeline should put a media item to the window just as other media sources and timelines do. This change provides a fake media item for both of them.
Further the MaskingMediaSource needs to provide a media item for when the real timeline of the masked media source is not available. This can be easily done by using mediaSource.getMediaItem() once available. For now a dummy is used to make ExoPlayerTest run green. This can be easily change to use mediaSource.getMediaSource as soon as this method is defined by the MediaSource interface.
PiperOrigin-RevId: 314897474
