The previous implementation hardcoded the X.509 KeyUsage extension to `keyCertSign` for all generated certificates. This was only correct for keys with the `ATTEST_KEY` purpose and violated the Android HAL specification for keys intended for other uses. For instance, a key created for signing (`KeyPurpose::SIGN`) requires the `digitalSignature` bit to be set, not `keyCertSign`.
This commit corrects the logic by dynamically constructing the `KeyUsage` bitmask from the key's specified purposes, adhering to the mapping defined in `KeyCreationResult.aidl`. This ensures that generated certificates now have the correct KeyUsage bits, accurately reflecting the key's intended function (e.g., signing, decryption, key wrapping) and making them compliant with the specification.
