We currently forget whether a source is seekable at re-preparation. This was
implemented intentionally this way under the assumption that we really can't seek
until we have loaded the seek map again. However, seek operations are only
allowed after a media period is prepared. So there is no harm in remembering
whether a source is seekable.
This problem currently prevents reusing ClippingMediaSources with
ExtractorMediaSource and a non-zero start clip position.
Issue: #5351
PiperOrigin-RevId: 229169441
66ca43ed1d