After enabling E2EE for the first time, keep the media transform
inplace, but make it a no-op.

This simplifies the flow a little since it doesn't require a media
session restart.

Way back when, there were performance concerns when having a transform,
which is what prompted us to implement the restar mechanism, but a lot
has changed since and both audio and video frames don't need to jump
threads these days. We are still not enabling an empty transform at all
times out of precaution.