[chromecast] Add dummy_external_audio_pipeline

Extracted dummy external audio pipeline implementation out of
libcast_external_audio_pipeline_1.0 target, so it can be used for
other targets

Bug: b/142420944
Change-Id: I727332c070c9ba8a3a850d1c0cb0800fe995a6d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917601
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715959}

chromecast/media/audio/BUILD.gn

@@ -86,7 +86,7 @@ if (is_android) {
   }
 }
 
-cast_shared_library("libcast_external_audio_pipeline_1.0") {
+source_set("dummy_external_audio_pipeline") {
   sources = [
     "external_audio_pipeline_dummy.cc",
   ]
@@ -96,6 +96,12 @@ cast_shared_library("libcast_external_audio_pipeline_1.0") {
   ]
 }
 
+cast_shared_library("libcast_external_audio_pipeline_1.0") {
+  public_deps = [
+    ":dummy_external_audio_pipeline",
+  ]
+}
+
 source_set("fake_external_audio_pipeline") {
   sources = [
     "fake_external_audio_pipeline.cc",