Fix potential double destroy of AssetManager am: 0a8a1e9d40

Change-Id: I10010fa2087e7978dc88b1d1e6f6e5d3a4661499

core/java/android/content/res/AssetManager.java

@@ -1157,8 +1157,11 @@ public final class AssetManager implements AutoCloseable {
             }
         }
 
-        if (mObject != 0) {
+        synchronized (this) {
-            nativeDestroy(mObject);
+            if (mObject != 0) {
+                nativeDestroy(mObject);
+                mObject = 0;
+            }
         }
     }