Police ATTEST_KEY purpose

The KeyMint spec has always required that keys with the ATTEST_KEY
purpose "must not have any other purpose".

Police this in the KeyMint reference implementation, for both generating
and importing keys.

Bug: 197096139
Test: VtsAidlKeyMintTargetTest
Change-Id: Ib1a4cf6a885aa3b54e23927ea779ce785a7326cc
1 file changed