fastboot: derive device locked state from Android property

We shall rely on Android property instead of parsing kernel
command line directly: parameters could be originated from
kernel command line or bootconfig.

Bug: 183237066
Bug: 183719613
Signed-off-by: Oleg Matcovschi <omatcovschi@google.com>
Change-Id: Ia70d286646a83a95967ca4293168a6550ffff7f0