Tweaks to fingerprint AIDL

1) Explicit numbering for Error enum, which contains intentionally
   skipped entries for backwards compatibility
2) Documentation for vendorCode. The vendorCodes must have a well
   defined behavior with the array API for things like GSI to work,
   e.g. generic system image with non-generic HAL implementation -
   with this explicit requirement, both components, as long as the
   contract is followed, would allow proper string feedback to be
   loaded/shown.

Bug: 168843220
Test: Builds
Change-Id: Ie78e42b926351a03162a4a6e40f3325641c34609