Add null check for the taken callback

The requestTetheredInterface() and releaseTetheredInterface() in
EthernetManager is annotated as @NonNull. Basically, the taken
callback parameter to the service implementation should also be
@NonNull. However, it still possible to use native commands to
call the method in the aidl. If a null callback is taken, it may
cause unexpected results and cause crashes. Add a null check and
throw NPE as a warning.

Bug: 190058445
Test: adb commands
Change-Id: I18fd63aba3f7326597fc3a8c93ba8c9097bf7348

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

@@ -35,6 +35,7 @@ import com.android.internal.util.IndentingPrintWriter;
 
 import java.io.FileDescriptor;
 import java.io.PrintWriter;
+import java.util.Objects;
 import java.util.concurrent.atomic.AtomicBoolean;
 
 /**
@@ -172,6 +173,7 @@ public class EthernetServiceImpl extends IEthernetManager.Stub {
 
     @Override
     public void requestTetheredInterface(ITetheredInterfaceCallback callback) {
+        Objects.requireNonNull(callback, "callback must not be null");
         NetworkStack.checkNetworkStackPermissionOr(mContext,
                 android.Manifest.permission.NETWORK_SETTINGS);
         mTracker.requestTetheredInterface(callback);
@@ -179,6 +181,7 @@ public class EthernetServiceImpl extends IEthernetManager.Stub {
 
     @Override
     public void releaseTetheredInterface(ITetheredInterfaceCallback callback) {
+        Objects.requireNonNull(callback, "callback must not be null");
         NetworkStack.checkNetworkStackPermissionOr(mContext,
                 android.Manifest.permission.NETWORK_SETTINGS);
         mTracker.releaseTetheredInterface(callback);