Don't process broadcasts until really ready.

Someone started setting mSystemReady too early, when we really want
to know SystemServer's real ready state.  Add a variable to track
that, and don't process broadcasts until that's set.

Bug: 78020762
Test: builds, boots
Change-Id: I65213e46044c95fb0a8a4b09b9aa463bb15c1844

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

@@ -282,7 +282,7 @@ public class NetworkStatsService extends INetworkStatsService.Stub {
     private Handler mHandler;
     private Handler.Callback mHandlerCallback;
 
-    private boolean mSystemReady;
+    private volatile boolean mSystemReady;
     private long mPersistThreshold = 2 * MB_IN_BYTES;
     private long mGlobalAlertBytes;