Adding public volume mounts for clone user.

Public SdCard Volumes are mounted only for user 0
(foreground user). This gives ENONT if the cloned
user tries to access the files in SdCard with
paths like "/storage/AB02-G212/DCIM/"

This change adds SdCard Volume mnt under
/mnt/usr/<cloned-user>/ which allows cloned apps
access to SdCard via direct file paths.

Bug: 203395175
Test: Manual by building and flashing device.
Change-Id: I091c40d3cb19915145cd5af40d1e79d5a9ecfa02
diff --git a/model/PublicVolume.h b/model/PublicVolume.h
index 3156b53..ca553b0 100644
--- a/model/PublicVolume.h
+++ b/model/PublicVolume.h
@@ -42,6 +42,8 @@
     explicit PublicVolume(dev_t device);
     virtual ~PublicVolume();
 
+    status_t bindMountForUser(userid_t user_id);
+
   protected:
     status_t doCreate() override;
     status_t doDestroy() override;