Moves socket_forward_proxy from commands/ to frontend/

Test: all good on gce-dev

Change-Id: Id9c60b0730a6ccba25cc8713f7dc7c7cb6db4415
diff --git a/common/Android.bp b/common/Android.bp
index 646ec16..2280c23 100644
--- a/common/Android.bp
+++ b/common/Android.bp
@@ -15,5 +15,6 @@
 
 subdirs = [
     "commands",
+    "frontend",
     "libs",
 ]