Revert isDefaultNetworkActive behavior when there is no default network

aosp/2605757 updated isDefaultNetworkActive returns false when there is
no default network.
But this change was not merged to U release branch and this change was
not well tested by dogfooding.
So this CL reverts the isDefaultNetworkActivity behavior change.

Bug: 279380356
Bug: 291870075
Test: atest FrameworksNetTests
Change-Id: Id88662faea9eeaba93d59ab2729f6204a3631ab1

service/src/com/android/server/ConnectivityService.java

@@ -11138,7 +11138,9 @@ public class ConnectivityService extends IConnectivityManager.Stub
                 new RemoteCallbackList<>();
         // Indicate the current system default network activity is active or not.
         // This needs to be volatile to allow non handler threads to read this value without lock.
-        private volatile boolean mIsDefaultNetworkActive;
+        // If there is no default network, default network is considered active to keep the existing
+        // behavior. Initial value is used until first connect to the default network.
+        private volatile boolean mIsDefaultNetworkActive = true;
         private final ArrayMap<String, IdleTimerParams> mActiveIdleTimers = new ArrayMap<>();
 
         private static class IdleTimerParams {
@@ -11321,8 +11323,9 @@ public class ConnectivityService extends IConnectivityManager.Stub
                     reportNetworkActive();
                 }
             } else {
-                // If there is no default network, default network is considered inactive.
-                mIsDefaultNetworkActive = false;
+                // If there is no default network, default network is considered active to keep the
+                // existing behavior.
+                mIsDefaultNetworkActive = true;
             }
         }