apt_key: Validate key_id and accept a leading '0x'

Accept a GPG key_id with a leading _0x_.

It's common to specify key is with a leading _0x_, it's an hexadecimal number after all. In such cases `apt_key` fails with _msg: key does not seem to have been added_.

This patch removes the leading _0x_ from the id (if present) and also validates that the id is a valid hex number.