Reland "Rename stereo video codec to multiplex"

This is a reland of bbdabe50db0cf09f6007dda12a6476dc4602b174.

Original change's description:
> Rename stereo video codec to multiplex
>
> This CL only does the rename from"stereo" to multiplex". With this we have a
> better name that doesn't clash with audio's usage of stereo.
>
> Bug: webrtc:7671
> Change-Id: Iebc3fc20839025f1bc8bcf0e16141bf9744ef652
> Reviewed-on: https://webrtc-review.googlesource.com/43242
> Commit-Queue: Emircan Uysaler <emircan@webrtc.org>
> Reviewed-by: Niklas Enbom <niklas.enbom@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#21769}

TBR=niklas.enbom@webrtc.org

Bug: webrtc:7671
Change-Id: I6f38dc46126f279f334d52b56339b40acdc30511
Reviewed-on: https://webrtc-review.googlesource.com/45820
Reviewed-by: Emircan Uysaler <emircan@webrtc.org>
Commit-Queue: Emircan Uysaler <emircan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21794}

common_types.h

diff --git a/common_types.h b/common_types.h
index 5f9d77a..bd23f0f 100644
--- a/common_types.h
+++ b/common_types.h
@@ -485,7 +485,7 @@
   kVideoCodecULPFEC,
   kVideoCodecFlexfec,
   kVideoCodecGeneric,
-  kVideoCodecStereo,
+  kVideoCodecMultiplex,
   kVideoCodecUnknown
 };