Use /mnt/storage for bind mounting package sandboxes.

Create a new slave bind mount at /mnt/storage and use this for
creating package sandboxes. This will help prevent package
specific data directories from getting duplicated and also make
handling bind mounts for packages with sharedUserIds easier.

Bug: 111890351
Test: manual
Change-Id: I0f590cc99a379d93f6db85bf56e8d274e3ea7488
diff --git a/VolumeManager.h b/VolumeManager.h
index dd7bf71..52203c5 100644
--- a/VolumeManager.h
+++ b/VolumeManager.h
@@ -179,6 +179,7 @@
 
     int mNextObbId;
     bool mSecureKeyguardShowing;
+    bool mMntStorageCreated;
 };
 
 #endif