fs_mgr: fs_mgr_overlayfs_teardown_one use .<hidden>

Switch to using a leading . to reference a teardown directory, and
if the top directory (/mnt/scratch/overlay) report ENOTEMPTY then
check if there is any non-hidden content, and if not then consider
removing the scratch partition.

Although not currently a problem, use basename of mount point in
fs_mgr_overlayfs_teardown_one when referencing the overlay directory
tree to match the setup behavior.  Would have become a problem if
we started dealing with sub-mount points (eg: /vendor/firmware_mnt).

Test: manual
Bug: 109821105
Change-Id: I05dafe29c9e376e8423aedc8783fe674a657b3ee