Improve unique attestation docs & tests

Improve the documentation and tests related to device-unique
attestation on StrongBox KeyMint devices:
* Test that the chain produced is exactly of length 2.
* Document how the chain needs to be structured.
* Explain the trust properties of the key used for the
  self-signed root.

Test: atest VtsAidlKeyMintTargetTest
Bug: 187803288
Change-Id: I09bb16d6938b567c114485d2df00bde9d3e1ccf9