Fix a timing issue where LauncherApps registers its callback twice

There is an edge case while the launcher process is killed and restarted by unexpected behavior.
LauncherApps unregisters a listener by being called back onCallbackDied from PackageCallbackList when the launcher process is killed.
However the launcher process is immediately restarted and registers a listener in the middle of processing binderDied() even before calling the onCallbackDied() in the binderDied().

This solution is to maintain a boolean, isWatchingPackageBroadcasts, determining whether or not the monitor has been registered.
startWatchingPackageBroadcasts() and stopWatchingPackageBroadcasts() are always called with the mListeners lock held, it can check/toggle the state safely in those methods.

Test: compile & verify basic functions working
Bug: 162753652
Change-Id: If91e4bc32b17b88576777699b3d8d9f409fae91d

services/core/java/com/android/server/pm/LauncherAppsService.java

@@ -75,6 +75,7 @@ import android.util.Log;
 import android.util.Pair;
 import android.util.Slog;
 
+import com.android.internal.annotations.GuardedBy;
 import com.android.internal.annotations.VisibleForTesting;
 import com.android.internal.content.PackageMonitor;
 import com.android.internal.os.BackgroundThread;
@@ -143,6 +144,9 @@ public class LauncherAppsService extends SystemService {
 
         private final MyPackageMonitor mPackageMonitor = new MyPackageMonitor();
 
+        @GuardedBy("mListeners")
+        private boolean mIsWatchingPackageBroadcasts = false;
+
         private final ShortcutChangeHandler mShortcutChangeHandler;
 
         private final Handler mCallbackHandler;
@@ -281,7 +285,10 @@ public class LauncherAppsService extends SystemService {
          * Register a receiver to watch for package broadcasts
          */
         private void startWatchingPackageBroadcasts() {
-            mPackageMonitor.register(mContext, UserHandle.ALL, true, mCallbackHandler);
+            if (!mIsWatchingPackageBroadcasts) {
+                mPackageMonitor.register(mContext, UserHandle.ALL, true, mCallbackHandler);
+                mIsWatchingPackageBroadcasts = true;
+            }
         }
 
         /**
@@ -291,7 +298,10 @@ public class LauncherAppsService extends SystemService {
             if (DEBUG) {
                 Log.d(TAG, "Stopped watching for packages");
             }
-            mPackageMonitor.unregister();
+            if (mIsWatchingPackageBroadcasts) {
+                mPackageMonitor.unregister();
+                mIsWatchingPackageBroadcasts = false;
+            }
         }
 
         void checkCallbackCount() {