Implement debugging local unlock keys in chrome://proximity-auth.

This CL also updates some older log messages to use the new PA_LOG macro, and removes a metadata check that is not being set on the GMS Core side.

BUG=409158
TEST=manual

Review URL: https://codereview.chromium.org/1264703003

Cr-Commit-Position: refs/heads/master@{#341634}