Fix unittest key path in Brillo.

The unittests weren't able to find the keys if it's not run from the
update_engine_unittests directory.

Test: /data/nativetest/update_engine_unittests/update_engine_unittests
Bug: 26955860

Change-Id: I4c189cca2714986d3d2b2669b54a5dff9a488964