fs_mgr_vendor_overlay: Mount vendor overlay with noatime

Change relatime to noatime as our filesystems and partitions are all
read-only, so relatime doesn't really work in the first place.

Test: atest -v fs_mgr_vendor_overlay_test
Test: Presubmit
Change-Id: I869bba24c2d6d8a986bf0eec0f487ba7b935ab4b