SDK / exoplayer

If keepalive is disabled the existing code over-eagerly releases
DrmSession instances. This is arguably OK since a (Default)DrmSession
should be released before its (Default)Manager is released
(since the underlying MediaDrm instance might be released when the
manager is released). And if all sessions are released before the
manager is released then `sessions` is empty, so the loop is a no-op.

Issue: #8576
PiperOrigin-RevId: 356955308