Start adbd in user mode if bootloader is unlocked

During automatic tests, we sometimes want to reboot the device out of
the rescue party remotely. And per http://go/recovery-adb-access, one
option is to start adbd in user build if the device has an unlocked
bootloader. This should not add more surface of attack. Because verified
boot is off with the unlocked bootloader, and the user can always flash
a custom recovery image that always starts adbd.

Bug: 141247819
Test: check adbd doesn't start in user build, unlock bootloader, and
check adbd starts.

Change-Id: I851746245f862cb4dfb01e6c3ad035f2c9f9ccec