cmd: avb: use unlocked state return by avb_ops->read_is_device_unlocked

use unlocked state to know if device is lock.
with android10 and new dunamic partition android have a userspace
implementation to flash parition, userspace fastboot[1] use
"androidboot.verifiedbootstate"[2] to know if device is lock or not.

[1] https://android.googlesource.com/platform/system/core/+/refs/heads/master/fastboot/device/utility.cpp#202
[2] https://source.android.com/security/verifiedboot/boot-flow#communicating-verified-boot-state-to-users



Signed-off-by: Guillaume La Roque <glaroque@baylibre.com>