Prevent channels being set on stopped transceiver.

Fixing bug that allows a channel to be set on a stopped transceiver.
This CL contains the following refactoring:
1. Extracted ChannelInterface from BaseChannel
2. Unified SetXxxMediaChannel (Voice, Video) into SetMediaChannel

Bug: webrtc:9932
Change-Id: I2fbf00c823b7848ad4f2acb6e80b1b58ac45ee38
Reviewed-on: https://webrtc-review.googlesource.com/c/110564
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Amit Hilbuch <amithi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25641}
diff --git a/pc/channel.cc b/pc/channel.cc
index 50fa567..0f857cd 100644
--- a/pc/channel.cc
+++ b/pc/channel.cc
@@ -702,7 +702,7 @@
       break;
     }
     case MSG_FIRSTPACKETRECEIVED: {
-      SignalFirstPacketReceived(this);
+      SignalFirstPacketReceived_(this);
       break;
     }
   }