SDK / exoplayer

Before this fix, if DefaultDrmSessionManager.release() was called while
there was at least one 'external' session reference still active (i.e.
session.referenceCount > 1) then the manager will release it's reference
immediately but when the session's reference count subsequently drops to
1 (due to external references being released) the manager will schedule
a task to release its internal reference *again*.

This change fixes the problem by only scheduling the timed release if
the manager is unreleased. This ensures that the internal references
are only released once.

Issue: #9193

PiperOrigin-RevId: 385580741