Small tweaks in LockSettingsService

Paves way for synthetic password flow: Two notable changes:
1. when unify/un-unify work challenges, provide the old work profile password.
2. when clearing lock, supply old credentials.

Test: Unit test to be added in a follow up CL.
Bug: 33126414
Change-Id: I2a9553c5e7cc701338436e99e5a1289cebd1eda9