SDK / exoplayer

It's a transient state whose only use is when passing a removed
download to onDownloadStateChanged. It's a bit strange to have it
as a proper state, since we end up asserting that it's an invalid
value in other places.

This change adds an explicit onDownloadRemoved, which allows
removal of the transient STATE_REMOVED, related assertions, and
uncertainty when dealing with an @State variable whether it's
necessary to handle it being STATE_REMOVED.

PiperOrigin-RevId: 242444128