do not sleep if it is shutting down

- Various sleep(5) for vold shutdown can increase shutdown time a lot.
- If it is shutting down, do not sleep at all. init will take care of
  active partitions if not unmounted.

bug: 64143519
Test: reboot and check logs from vold, check if "ShutdownThread: Shutdown wait timed out" happens.
Change-Id: I7cb91427ad2205fe23a054d255caf7ffdfd9f6c3
diff --git a/VolumeManager.h b/VolumeManager.h
index 796a91d..537aebe 100644
--- a/VolumeManager.h
+++ b/VolumeManager.h
@@ -70,6 +70,9 @@
     static const char *ASECDIR;
     static const char *LOOPDIR;
 
+    //TODO remove this with better solution, b/64143519
+    static bool shutting_down;
+
 private:
     static VolumeManager *sInstance;