Split iOS sdk in to separate targets

This CL splits the iOS sdk into separate static libraries for video,
audio, ui, common, and peerconnection-related code. This will in the
future make it easier to compile WebRTC without unneeded components.

BUG=webrtc:4867

Review-Url: https://codereview.webrtc.org/2862543002
Cr-Commit-Position: refs/heads/master@{#18166}

webrtc/modules/video_coding/BUILD.gn

@@ -376,7 +376,7 @@ if (rtc_include_tests) {
     if (is_ios || is_mac) {
       deps += [
         "../../media:rtc_media_base",
-        "../../sdk:webrtc_h264_video_toolbox",
+        "../../sdk:objc_videotoolbox",
       ]
     }
   }