Zeroize the vendor auth secret encryption key

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: I76f70b4df6f8bbee0018fb6ee6ab7d9c56fe9065