Zeroize password metrics key in savePasswordMetrics()

Secrets derived from or unlocked by the LSKF need to be zeroized as soon
as they are no longer needed.

Bug: 320392352
Test: atest FrameworksServicesTests:com.android.server.locksettings
Flag: EXEMPT bugfix
Change-Id: Ia21653380af5fb92e12514295d3dd988bd83e42d