Disable darwin build of webrtc

libopus and libvpx are disabled on darwin.

Bug: 162102724
Test: TH
Change-Id: Id8787e81058053da18b43a85b3d6ececbaa3af91
diff --git a/Android.bp b/Android.bp
index 24e8e6a..14d638b 100644
--- a/Android.bp
+++ b/Android.bp
@@ -49,6 +49,11 @@
             enabled: false,
         },
     },
+    target: {
+        darwin: {
+            enabled: false,
+        },
+    },
 }
 
 cc_library_static {