fs_mgr: overlayfs deal with verity detection issues

Stop checking verity on each entry in the fstab, collect the verity
list whole and use in the loop.  Open code verity detection for
"system" for cases where device table is incompletely specified, does
not specify a "/" or a "/system" entry.

Test: manual, /system on system-as-root overlays correctly.
Bug: 109821005
Change-Id: I7bc5fbef6577bb0823cb5deb26c370219e4e597b