Zeroize intermediate key in transformUnderWeaverSecret()

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: If47d54f2a94ec55641958f64fadd542ec79b46dc