Hide the internal AudioEncoderOpus class by giving it an "Impl" suffix

We've done this previously with the other audio encoders, but Opus had
to wait until all external users had been updated.

BUG=webrtc:7847

Change-Id: I70422d7b6c715f32a43bee88febcf6b6155e18b3
Reviewed-on: https://webrtc-review.googlesource.com/8000
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20424}
diff --git a/api/audio_codecs/opus/BUILD.gn b/api/audio_codecs/opus/BUILD.gn
index faa5956..9d07c95 100644
--- a/api/audio_codecs/opus/BUILD.gn
+++ b/api/audio_codecs/opus/BUILD.gn
@@ -30,12 +30,16 @@
 }
 
 rtc_source_set("audio_encoder_opus") {
-  sources = [
+  public = [
     "audio_encoder_opus.h",
   ]
+  sources = [
+    "audio_encoder_opus.cc",
+  ]
   deps = [
     ":audio_encoder_opus_config",
     "..:audio_codecs_api",
+    "../..:optional",
     "../../../modules/audio_coding:webrtc_opus",
     "../../../rtc_base:rtc_base_approved",
   ]