Zeroize intermediate key in stretchedLskfToWeaverKey()

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: I7083e1b86e7c6b1bf5bb5867867479988d60013e