Start THM outside of the frp block

Starting TestHarnessModeService outside of the FRP block will allow OEMs
to provide their own implementation of
PersistentDataBlockManagerInternal in LocalServices, which will allow
them to satisfy all CTS requirements even without supporting Factory
Reset Protection.

Bug: 131439285
Test: make && adb shell cmd testharness enable
Change-Id: If3ea192a22105716cab1dbed832f8c20b51058b7