Move MockVideoDecoder to api/test.

Move MockVideoDecoder from
modules/video_coding/include/mock/mock_video_codec_interface.h
to
api/test/mock_video_decoder.h

The mock encoder has already moved:
https://webrtc-review.googlesource.com/c/src/+/105620

Keeping the old header until downstream projects have been updated.

Bug: webrtc:9722
Change-Id: I4bc849173a04813064212f17761876695ca3fed4
Reviewed-on: https://webrtc-review.googlesource.com/c/105900
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25170}

modules/video_coding/include/mock/mock_video_codec_interface.h

@@ -19,6 +19,8 @@
 
 namespace webrtc {
 
+// NOTE: Deprecated file, include api/mock_video_(encoder|decoder).h instead.
+
 class MockEncodedImageCallback : public EncodedImageCallback {
  public:
   MOCK_METHOD3(OnEncodedImage,