KeyMint: check length of verified boot key

Only police for future versions of KeyMint, so that any existing
implementations that use the full key don't suddenly start to fail
VTS.

Bug: 305103215
Test: VtsAidlKeyMintTargetTest
Change-Id: If6534b84b6eff8cdb281586e17a5f89c7bf5f5d0
2 files changed