Add logging to NetworkStackClient

The current logging relies on the system log that may rotate. This is a
problem to investigate bugs where the network stack service is not
started. Add a SharedLog to NetworkStackClient and dump it in
ConnectivityService (for lack of a better place to dump it).

Bug: 124307802
Test: Booted, dumpsys connectivity shows the log
Change-Id: If596451d0613645f68a8a32acb032eea76ea506c

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

@@ -2373,6 +2373,11 @@ public class ConnectivityService extends IConnectivityManager.Stub
 
             pw.decreaseIndent();
         }
+
+        pw.println();
+        pw.println("NetworkStackClient logs:");
+        pw.increaseIndent();
+        NetworkStackClient.getInstance().dump(pw);
     }
 
     private void dumpNetworks(IndentingPrintWriter pw) {