TestHarnessModeService uses MainUser

Previously, this service dealt with user 0's screen lock (etc.). For
devices in Headless System User Mode, this should instead be the
MainUser, which will be different from user 0 on such devices.

Test: ran "adbshell cmd testharness enable" and confirmed: If MainUser has a password, it returns an error. Otherwise, it proceeds.
Bug: 265213225
Change-Id: Ie73fcab156260dc869504c803c81e8c0ff1b2d0c