`DefaultDrmSession(Manager)` expect most of their methods to be called
on the 'playback thread'. There isn't a playback thread in the case of
`OfflineLicenseHelper`, but in that case it's the thread backing
`DefaultDrmSessionManager.playbackLooper`, which is `OfflineLicenseHelper.handlerThread`.
PiperOrigin-RevId: 520053006
(cherry picked from commit 2f7ba44e)
