Make fs_mgr_overlayfs_mount_fstab_entry() available for user builds

Rename fs_mgr_overlayfs_mount_fstab_entry() to
fs_mgr_mount_overlayfs_fstab_entry() and move it out of
fs_mgr_overlayfs.cpp to make it available for user builds.

Add checks to unsure overlayfs mount point doesn't contain symbolic
link or /../.

Check the mount point with an allowlist if user build. The mount point
should either be /vendor, /product ... or their submounts, or strict
submounts of /mnt/vendor and /mnt/product.

Bug: 188862155
Test: Boot test with overlayfs mount entries on user build
Change-Id: I3b60dfa4b63cf2ae0754f53d1d08365aa7be1ee0
Merged-In: I3b60dfa4b63cf2ae0754f53d1d08365aa7be1ee0
(cherry picked from commit 23816e84)