One of the PhoneLab participants accidentally forgot the PIN code for his phone, thus can not access the phone at all. There are tremendous tutorials online on how to solve this. This is what I tested and worked.

Since PhoneLab devices are flashed with Clockworkmod recovery, I reboot the phone (Nexus 5) into recovery mode, mount /system and /data partitions, adb shell into the phone, and delete these two files: /data/system/password.key and /data/system/gesture.key. Then the problem was fixed: no lock screen after powering on the phone.

These are two methods that I tried yet failed. Both have something to do with the settings.db file.

adb shell
cd /data/data/
sqlite3 settings.db
update system set value=0 where name='lock_pattern_autolock';
update system set value=0 where name='lockscreen.lockedoutpermanently';

And this one (from xda-developers forum):

adb shell
sqlite3 /data/data/
sqlite> update secure set value=65536 where name='lockscreen.password_type';
sqlite> .exit
# exit
adb reboot