KeyMint: improve HAL spec and tests

- clarify & test BIGNUM spec
- allow alternative return codes when requesting device unique
  attestation
- use specific error for early boot import failure
- test more early boot key scenarios (in post-early-boot mode)

Test: VtsAidlKeyMintTargetTest
Change-Id: I70a342084a29144aef1ed0ff80fec02cc06ffbc0