Enforce ACCESS_NETWORK_STATE to getAvailableInterfaces()

getAvailableInterfaces() will return available ethernet
interfaces which are the information about networks. So it should
enforce ACCESS_NETWORK_STATE permission check to ensure the
applications are allowed to access the information.

Bug: 174573778
Test: TetheringTests
Test: CtsTetheringTest
Change-Id: I7aaa5225d56f2feecc51ba263489ed0ce02fd651

service-t/src/com/android/server/ethernet/EthernetServiceImpl.java

@@ -87,6 +87,8 @@ public class EthernetServiceImpl extends IEthernetManager.Stub {
 
     @Override
     public String[] getAvailableInterfaces() throws RemoteException {
+        enforceAccessPermission();
+
         return mTracker.getInterfaces(checkUseRestrictedNetworksPermission());
     }