SDK / exoplayer

*** Original commit ***

Ensure MediaSourceFactory instances can be re-used

This fixes DefaultDrmSessionManager so it can be used by a new Player
instance (by nulling out its reference to the playback thread, which is
unique per-Player instance). This only works if the
DefaultDrmSessionManager is 'fully released' before being used by the
second Player instance, meaning that the reference count of the manager
and all its sessions is zero.

#exofixit
Issue: #9099

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PiperOrigin-RevId: 396861138