Introduce LockscreenCredential to LockSettingsService

Use LockscreenCredential as the representation format for credentials
throughout the LSS system service, until when the raw credential bits
are needed. Most of the changes are just mechanical, except in places
when null was used to represent empty credential, but now it has its
own concrete object.

Test: atest com.android.server.locksettings
Test: atest com.android.server.devicepolicy.DevicePolicyManagerTest
Test: atest MixedManagedProfileOwnerTest#testResetPasswordWithToken
Test: atest com.android.cts.devicepolicy.PasswordComplexityTest
Test: atest com.android.cts.devicepolicy.ManagedProfilePasswordTest
Bug:65239740
Change-Id: I3997c3c2b6651d11a0e447448ac3a8523a18fa36