Implement fake keystore for LockSettingsService unit tests.

This allows for working encryption in unit tests.

Remove unnecessary overrides in LockSettingServiceTestable for
tieProfileLockToParent and getDecryptedPasswordForUnifiedProfile

Add necessary set-up logic in SyntheticPasswordTests to initialize
SP for child profiles alongside parent so that they're not treated
as special users in SoftwareRateLimiter due to a null protectorId.

Bug: 412331826
Flag: EXEMPT PURE_REFACTOR
Test: atest FrameworksServicesTests:com.android.server.locksettings
Change-Id: I4d06493f1362d9af5fd9c0ac9631ee0c1320f6f6