Mount tmpfs on external CE and DE storage

Before, we mount tmpfs on /mnt/expand, which is incorrect as
there were other directories which shouldn't be overlayed.

Now, we only mount tmpfs on CE and DE dir under /mnt/expand.

Also, set the correct selinux file label back to CE and DE directories.

Test: Labels are correctly labeled
Test: Only CE and DE dir are mounted under /mnt/expand

Bug: 143937733
Change-Id: I6dbff432552ba85c2f878b25cb1a85ccfa6bfa50