Do not re-initialize synthetic password

A bug was introduced in R where LSS ends up regenerating SP
when an escrow token is being auto-activated on unsecured user,
due to a logic error in shouldMigrateToSyntheticPasswordLocked().
Fix the bug and add some safeguards as well as unit test to
prevent future regressions.

Bug: 168692734
Test: atest com.android.server.locksettings
Change-Id: If35f2fd26b49faf6e3d0d75c10b1b3bb95f247c2