umount_all is the cleanup step for mount_all.
In particular, the mount_all builtin creates a verity device,
'postinstall-verity', for the following line:
system /postinstall ... ... slotselect_other,logical,avb_keys=...
cppreopt umounts /postinstall but doesn't destroy the postinstall-verity
device, causing OTA to fail (because it cannot destroy the
system_[other] device). umount_all also destroy the verity device.
Note that mount_all does not map system_[other]; it is mapped by
first stage init. Hence, umount_all doesn't destroy it either. The OTA
client is reponsible for unmapping the device itself.
Bug: 129988285
Test: flash, boot, then check `dmctl list devices`, then OTA
Change-Id: Id3ab65b3860b6ea6cfec310ab13652009c81f415
Merged-In: Id3ab65b3860b6ea6cfec310ab13652009c81f415
402633d605