fs_mgr_overlayfs: Trim surrounding "@" from the per mountpoint scratch dir name

Right now we encode the per mountpoint scratch dir name like this:
  /system -> /mnt/overlay/@system/
  /product/app -> /mnt/overlay/@product@app/

This CL changes it to:
  /system -> /mnt/overlay/system/
  /product/app -> /mnt/overlay/product@app/

This makes it so that the encoded path for top-level mountpoints (like
/system, /vendor) would have the same encoded scratch dir as before
https://r.android.com/2795755 was introduced.
With this change old first-stage-init can handle top-level remounts
correctly. However for mountpoints with '/' in them, their remount
scratch dirs would be encoded with the new format, and old
first-stage-init would ignore and not setup these during boot.

This makes the remount mechanism to function partially when running on
an old ramdisk (first-stage-init) + new system combo.
Normally we expect the init_boot ramdisk to be upgraded alongside
system.img, so this change isn't strictly needed. However there are
cases where we might want to develop new OS features on old vendor
platform, thus this change.

Bug: 306124139
Bug: 243503963
Test: adb-remount-test
Change-Id: I9b43641bb338f11c6c83888880948e4b85af14e1