Mark PDB identifier as SystemApi

Had already marked the API itself as system api, just not
the Context.BLAH id. Also update the javadoc as it doesn't
return an FD anymore.

Change-Id: Ic8a5b8c75b0ba0bdf4ad26cf33d5f87eea568376

Author: Andres Morales

core/java/android/content/Context.java

@@ -2883,12 +2883,14 @@ public abstract boolean startInstrumentation(@NonNull ComponentName className,
 
     /**
      * Use with {@link #getSystemService} to retrieve a {@link
-     * android.service.persistentdata.PersistentDataBlockManager} instance retrieving
-     * a file descriptor for a persistent data block.
+     * android.service.persistentdata.PersistentDataBlockManager} instance
+     * for interacting with a storage device that lives across factory resets.
+     *
      * @see #getSystemService
      * @see android.service.persistentdata.PersistentDataBlockManager
      * @hide
      */
+    @SystemApi
     public static final String PERSISTENT_DATA_BLOCK_SERVICE = "persistent_data_block";
 
     /**