KeyMint VTS: cope with ATTEST_KEY +/- SIGN

This change is a back-port of http://aosp/1877245 but adapted to allow
for devices that passed the original (but incorrect) version of the VTS
tests.

Bug: 197096139
Bug: 230074335
Test: VtsAidlKeyMintTargetTest
Change-Id: I447e6dc9dc049922725160c48c2406d79a8cffb5