Add proguard rules for TcUtils

Without this, proguard strips tcFilterAddDevIngressPolice (as its unused
inside the Tethering apk) and crashes the system.

Bug: 202086915
Bug: 157552970
Test: device boots
Change-Id: I3f29a2d2371ecbfad929f8812f7f15f68258b5d1
diff --git a/Tethering/proguard.flags b/Tethering/proguard.flags
index f62df7f..6735317 100644
--- a/Tethering/proguard.flags
+++ b/Tethering/proguard.flags
@@ -8,6 +8,10 @@
     native <methods>;
 }
 
+-keep class com.android.networkstack.tethering.util.TcUtils {
+    native <methods>;
+}
+
 -keepclassmembers public class * extends com.android.networkstack.tethering.util.Struct {
     *;
 }