Allow Test Harness Mode when secure ADB is disabled

Some builds may not have secure ADB (ro.secure.adb=1) enabled; however,
they should still be able to use Test Harness Mode. This enables those
builds to enable Test Harness Mode even if they're running the baked-in
ADB keys and/or they don't require keys at all.

BUG=150721793

Change-Id: If095c379f70a6ca7b0cf3be8b30dfa43b85633e3