Fix bug in VTS attestation cert verification.

Keymaster VTS is failing to verify that the last certificate in the
chain is self-signed.  CTS and GTS tests verify this, but it should be
validated at this level as well.

Bug: 79123157
Test: VtsHalKeymasterV3_0TargetTest
Change-Id: I5ff33fc8186182c2cf8d43d90cd59f89ce45d416