Fix disable-verity when the underlying block device is RO

If verity is enabled and the underlying block device is marked
read-only, disable-verity fails. We cannot use the existing code
for enable-verity to make the device writable as the device in
/proc/mounts will be the verity device instead of the underlying
device we want to change. This change makes the correct device
writable when altering verity state.

Change-Id: I423ee50fb34d78cff2fe843318b9081c03c5142d