Zeroize metrics key in getPasswordMetrics()

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: Ia1bee11395fa2bcf470ee1db797859beb5e62866