SDK / exoplayer

This change introduces a third 'state' for `DefaultDrmSessionManager`:
It's been fully released (prepareCount == 0) but at least one of its
sessions is still active.

In this state new acquisitions are rejected (`(pre)acquireSession()`
calls will fail) but the machinery to support the existing sessions
(ExoMediaDrm and MediaDrmHandler) is kept until they're all released.

This change will allow us to remove the TODO in MediaCodecRenderer
that resolves Issue: #8842.

PiperOrigin-RevId: 376193952