Simplify ChannelManager initialization.

* A ChannelManager instance is now created via ChannelManager::Create()
* Initialization is performed inside Create(), RAII.
* All member variables in CM are now either const or RTC_GUARDED_BY
  the worker thread.
* Removed dead code (initialization and capturing states are gone).
* ChannelManager now requires construction/destruction on worker thread.
  - one fewer threads that its aware of.
* media_engine_ pointer removed from ConnectionContext.
* Thread policy changes moved from ChannelManager to ConnectionContext.

These changes will make a few other issues easier to fix, so tagging
those bugs with this CL.

Bug: webrtc:12601, webrtc:11988, webrtc:11992, webrtc:11994
Change-Id: I3284cf0a08c773e628af4124e8f52e9faddbe57a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/212617
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33614}

media/engine/null_webrtc_video_engine_unittest.cc

@@ -41,8 +41,7 @@ TEST(NullWebRtcVideoEngineTest, CheckInterface) {
 
   CompositeMediaEngine engine(std::move(audio_engine),
                               std::make_unique<NullWebRtcVideoEngine>());
-
-  EXPECT_TRUE(engine.Init());
+  engine.Init();
 }
 
 }  // namespace cricket