KeyMint VTS: don't combine ATTEST_KEY with SIGN

Update the VTS tests so that attestation keys are not created with
another purpose.

Bug: 197096139
Test: VtsAidlKeyMintTargetTest
Merged-In: Ib6e4ad98cbe5c3015138854679b11fa0e683ade9
Change-Id: Ib6e4ad98cbe5c3015138854679b11fa0e683ade9
Ignore-AOSP-First: cross-merge from aosp/master