Add @GuardedBy annotation to PersistentDataBlockService#mIsWritable.

Change-Id: I1024f2a56badde5c123d025d6fe02f42559cbcb1
Test: manual
Bug: 30352311
(cherry picked from commit f6f1d62748)

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

@@ -29,6 +29,7 @@ import android.service.persistentdata.IPersistentDataBlockService;
 import android.util.Slog;
 
 import com.android.internal.R;
+import com.android.internal.annotations.GuardedBy;
 
 import libcore.io.IoUtils;
 
@@ -76,6 +77,8 @@ public class PersistentDataBlockService extends SystemService {
 
     private int mAllowedUid = -1;
     private long mBlockDeviceSize;
+
+    @GuardedBy("mLock")
     private boolean mIsWritable = true;
 
     public PersistentDataBlockService(Context context) {