Test validity of device-unique attestation chain

Test the validity of the chain produced when device-unique attestation
is requested.

When the caller requests that the key attestation be signed using the
device-unique attestation key, the chain will look different than
a chain signed by the batch key (common case):
(1) The chain is exactly of length 2.
(2) The root is self-signed and is unique to the device.

Test that the chain is correctly signed in this change. The root is not
currently correctly self-signed, so don't test (2) yet.

Bug: 189425310
Bug: 187803288
Test: atest VtsHalKeymasterV4_1TargetTest:PerInstance/DeviceUniqueAttestationTest
Ignore-AOSP-First: Already merged in aosp
Merged-In: I91578eb2b7588685cc86c467423e9394c3f3c262
Change-Id: I7e83ba7c9c8c68b95b1456fb37bb5aa939c4e6f8