Leave NetworkStatsService around during shutdown.

We've seen system code continue to ask about network statistics
during a shutdown, so keep our objects around to avoid crashing
with NPEs.

Test: builds, boots
Bug: 73323432
Change-Id: I2654d37a59eb830ff32a2151ae6ad6a999a41677

services/core/java/com/android/server/net/NetworkStatsService.java

@@ -422,13 +422,6 @@ public class NetworkStatsService extends INetworkStatsService.Stub {
         mUidRecorder.forcePersistLocked(currentTime);
         mUidTagRecorder.forcePersistLocked(currentTime);
 
-        mDevRecorder = null;
-        mXtRecorder = null;
-        mUidRecorder = null;
-        mUidTagRecorder = null;
-
-        mXtStatsCached = null;
-
         mSystemReady = false;
     }