Revert "ConnectionContext: remove media engine without blocking."

This reverts commit 2ba941e6bc1d20acb9cfda4b87ba53c80640bbcb.

Reason for revert: Temporarily reverting due to b/269628432.

Original change's description:
> ConnectionContext: remove media engine without blocking.
>
> Bug: webrtc:14449
> Change-Id: I445114c14f4d440a5a8cac003266047fe4588dab
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/288580
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Commit-Queue: Markus Handell <handellm@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#38928}

Bug: webrtc:14449
Change-Id: If2f23662e486a1c1f85c318fc98c441aab9ace31
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/295862
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Markus Handell <handellm@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39454}