Improve watchdog explanation when system server is blocked

Change-Id: I5965a2b01c474cbe2a1ab342c3520b3d403d92e8
diff --git a/services/java/com/android/server/Watchdog.java b/services/java/com/android/server/Watchdog.java
index de941f2..d4d3850 100644
--- a/services/java/com/android/server/Watchdog.java
+++ b/services/java/com/android/server/Watchdog.java
@@ -424,7 +424,7 @@
                 }
 
                 name = (mCurrentMonitor != null) ?
-                    mCurrentMonitor.getClass().getName() : "null";
+                    mCurrentMonitor.getClass().getName() : "main thread blocked";
             }
 
             // If we got here, that means that the system is most likely hung.