SyncSM10: explicitly keep TetheringUtils jni methods

Found R8 accidentally remove TetheringUtils jni methods while doing
some refactor. If any of jni java methods is removed by R8, devices
would fail to boot because of jni registering failure. Explicitly
adding progurad rule to make sure R8 never remove it.

Test: boot, atest CtsTetheringTest

Change-Id: I091a2f094d01f683fb454ed4624be35555940393

Tethering/proguard.flags

@@ -15,6 +15,10 @@
     native <methods>;
 }
 
+-keep class com.android.networkstack.tethering.util.TetheringUtils {
+    native <methods>;
+}
+
 # Ensure runtime-visible field annotations are kept when using R8 full mode.
 -keepattributes RuntimeVisibleAnnotations,AnnotationDefault
 -keep interface com.android.networkstack.tethering.util.Struct$Field {