Add @GuardedBy annotation to PersistentDataBlockService#mIsWritable.

Change-Id: I1024f2a56badde5c123d025d6fe02f42559cbcb1
Test: manual
Bug: 30352311
(cherry picked from commit f6f1d627483b4dad9d65176769a1ee92c59a4810)
diff --git a/services/core/java/com/android/server/PersistentDataBlockService.java b/services/core/java/com/android/server/PersistentDataBlockService.java
index 32b183b..9346083 100644
--- a/services/core/java/com/android/server/PersistentDataBlockService.java
+++ b/services/core/java/com/android/server/PersistentDataBlockService.java
@@ -29,6 +29,7 @@
 import android.util.Slog;
 
 import com.android.internal.R;
+import com.android.internal.annotations.GuardedBy;
 
 import libcore.io.IoUtils;
 
@@ -77,6 +78,8 @@
 
     private int mAllowedUid = -1;
     private long mBlockDeviceSize;
+
+    @GuardedBy("mLock")
     private boolean mIsWritable = true;
 
     public PersistentDataBlockService(Context context) {