Zeroize keystore password in unlockKeystore()

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: I82deeeb5aeaea5a83a0d6a0475a5df9d813764eb