LockSettingsService: migrate to its own keystore namespace

Moves keystore keys used by LockSettingsService to a separate
namespace, migrating existing key during boot if needed.
Affected keys are encryption keys for synthetic password blob,
unified profile challenge encryption key and unified profile
challenge cache key.

Bug: 184664830
Test: atest com.android.server.locksettings
Change-Id: Iac339e64812232875afa9dc6109b848b50b0ef2f