This change uses this new method everywhere we currently `instanceof`
check an `Extractor` directly. This allows us to introduce
wrapping/delegating `Extractor` instances - because the `instanceof`
checks will continue to operate on the underlying instance.
HLS is a slightly different case, because it directly re-instantiates
`Extractor` instances, which is not compatible with an arbitrary
wrapping structure. Luckily the only `Extractor` instances that HLS
re-instantiates do not support muxed subtitles, so won't be wrapped
in the first place (although future changes might use the
delegating-`Extractor` pattern for other purposes, which might affect
HLS).
PiperOrigin-RevId: 542550928
bc06039f7f