LockSettingsService: Authsecret HIDL->AIDL adapter

Simplify LockSettingsService by hiding the details of supporting the
legacy HIDL interface for IAuthSecret inside an adapter that presents
the AIDL interface. Refactor to clear the way for b/258560859

Bug: 244746589
Bug: 258560859
Test: atest com.android.server.locksettings
Change-Id: I3582ce5468b577289634819e75db548455fc9c45