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}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index 0727d82..ac3e308 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -517,7 +517,24 @@
     ]
   }
 
+  rtc_source_set("mock_video_decoder") {
+    visibility = [ "*" ]
+
+    testonly = true
+    sources = [
+      "test/mock_video_decoder.cc",
+      "test/mock_video_decoder.h",
+    ]
+
+    deps = [
+      "../api/video_codecs:video_codecs_api",
+      "../test:test_support",
+    ]
+  }
+
   rtc_source_set("mock_video_encoder") {
+    visibility = [ "*" ]
+
     testonly = true
     sources = [
       "test/mock_video_encoder.cc",