fs_mgr: Fix disable-verity for verifyatboot partitions

This allows a remount to be performed on partitions which have been
verified at boot, without causing the verity service to complain of
"corrupted" blocks that could have been modified as a result of a
remount (while the verity checksum remained unchanged).

Bug: 32638755
Change-Id: I77cf7dd20ee5c5f5dac80f73f292e32583fe5906