Replace user with debuggable check in ConnectivityService

IS_USER is a hidden symbol, whereas IS_DEBUGGABLE is part
of the module API.
The check is generally equivalent, as non-user builds are eng or
userdebug, which are debuggable.

Bug: 170598012
Change-Id: I7d09939652844eb2e3a6f0200b1ba50691a147da
Test: atest FrameworksNetTests

services/core/java/com/android/server/ConnectivityService.java

@@ -7233,7 +7233,7 @@ public class ConnectivityService extends IConnectivityManager.Stub
         }
 
         void addRequestReassignment(@NonNull final RequestReassignment reassignment) {
-            if (!Build.IS_USER) {
+            if (Build.IS_DEBUGGABLE) {
                 // The code is never supposed to add two reassignments of the same request. Make
                 // sure this stays true, but without imposing this expensive check on all
                 // reassignments on all user devices.