fs_mgr: allow disable dm-verity when the device is unlocked without metadata

To start Treble VTS test, a single AOSP system.img will be flashed onto
the device. The size of AOSP system.img might be different than the
system partition size on device, making locating verity metadata fail
(at the last fixed size of the partition).

This change allows disabling dm-verity on system partition when the
device is unlocked (orange device state) with invalid metadata.

BUG: 35603549

Test: Boot bullhead with a different-sized system.img, checks verity is
      not enabled via:

          "adb shell getprop | grep partition.system.verified"

Change-Id: Id89d023e82c78ff1902ab7a528431cf7f65e49ad