KeyMint: Fix device-unique attestation chain specification

Fix the device-unique attestation chain specification: The chain should
have two or three certificates.
In case of two certificates, the device-unique key should be used for
the self-signed root.
In case of three certificates, the device-unique key should be certified
by another key (ideally shared by all StrongBox instances from the same
manufacturer, to ease validation).

Adjust the device-unique attestation tests to accept two or three
certificates in the chain.

Additionally, the current StrongBox KeyMint implementation can not yet
generate fully-valid chains (with matching subjects and issuers), so
relax that check.

Bug: 191361618
Test: m VtsAidlKeyMintTargetTest
Change-Id: I6e6bca33ebb4af67cac8e41a39e9c305d0f1345f