Don't create channel_manager when media_engine is not set

Also remove a bunch of functions in ChannelManager that were just
forwarding to MediaEngineInterface.

Bug: webrtc:13931
Change-Id: Ia38591fd22c665cace16d032f5c1e384e413cded
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261304
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36801}

pc/rtp_transceiver_unittest.cc

@@ -126,7 +126,7 @@ class RtpTransceiverUnifiedPlanTest : public ::testing::Test {
                 rtc::Thread::Current(),
                 receiver_),
             &channel_manager_,
-            channel_manager_.GetSupportedAudioRtpHeaderExtensions(),
+            channel_manager_.media_engine()->voice().GetRtpHeaderExtensions(),
             /* on_negotiation_needed= */ [] {})) {}
 
   static rtc::scoped_refptr<MockRtpReceiverInternal> MockReceiver() {