Add attestation format documentation

Somehow the attestation format docs got dropped from KeyMint in the
transition from Keymaster.  This replaces them, and also clarifies
that KeyMint StrongBox should not support device attestation.

Test: VtsAidlKeyMintTargetTest
Change-Id: I2334e99b4797c7a0e2e59727ffa730cf7504df31
4 files changed