Accommodate different at-vboot-state output format in at_auth_unlock.py

The format of the output for 'fastboot getvar at-vboot-state' varies
slightly between SoC vendors. Update the script to accommodate the
differences.

Bug: 111096941
Bug: 112481326
Test: Verified unlock works as expected on hardware from different SoC
vendors.

Change-Id: I5a80418f1464da4226b78b88b5b3706c2b0f845c