Currently acquireSession() fails with an NPE from
checkNotNull(exoMediaDrm). A follow-up change will result in exoMediaDrm
sometimes being non-null while prepareCount==0 (and in this case we
still want acquireSession() to fail).
preacquireSession() doesn't currently fail in a way the caller can
observe - the same NPE is thrown, but asynchronously and it doesn't
propagate out of the background thread. Throwing directly seems
preferable since it's a clear bug to be trying to preacquire sessions
from an unprepared/released manager.
PiperOrigin-RevId: 375906450
