Fix proguard rules for Struct in NetworkStack

All members of the Struct class need to be kept, not just the constructor.

Change-Id: I8705234eb2608c0dfe324c517b8885a93271b6c2
Test: m
diff --git a/Tethering/proguard.flags b/Tethering/proguard.flags
index 9ab56c2..75ecdce 100644
--- a/Tethering/proguard.flags
+++ b/Tethering/proguard.flags
@@ -9,7 +9,7 @@
 }
 
 -keepclassmembers public class * extends com.android.networkstack.tethering.util.Struct {
-    public <init>(...);
+    *;
 }
 
 -keepclassmembers class android.net.ip.IpServer {