Store LockscreenCredential in non-movable byte array

As another small step towards making LSKF-related secrets be zeroized
reliably, allocate the byte[] of LockscreenCredential as non-movable.

Bug: 320392352
Test: atest FrameworksCoreTests:LockscreenCredentialTest
Test: atest FrameworksServicesTests:com.android.server.locksettings
Flag: android.security.secure_array_zeroization
Change-Id: Ic106bf1864d64f8afb2c2e5cb88a82cd95292804