Read Verified Boot key from system property in default KeyMint implementation.

Even though this (insecure) implementation technically has access to
the Verified Boot key on unlocked builds, it should only send this
value if the bootloader is locked (in order to pass the VTS-16+ test
that checks that the attested "rootOfTrust.verifiedBootKey" value is
32 bytes of zeroes on unlocked devices).

Bug: 220834466
Test: atest VtsAidlKeyMintTargetTest (uses the KeyMint I'm updating)
Change-Id: I30e82165792facef85e9d66d4e6ebe8f64e1086b