identity: Check freshness of verification token in TA.

A problem where credstore didn't always use the TA-generated challenge
in the verification token was fixed in aosp/1619825. With this bug-fix
we can now reliably check that the passed-in verification token is
always fresh.

Bug: 181893400
Test: atest android.security.identity.cts on emulator
Test: atest VtsHalIdentityTargetTest
Change-Id: Iffdf026475da6321764561972aae27a82ab94530