aip.updated model property is not consistent with markdown front-matter

[eljobe]

For example, look at the latest change to aip-162: aip-dev/google.aip.dev@e487231

In that change the author updated the updated field on line 3 of 0162.md

But, the had no impact on the date displayed in the generated HTML.

It looks like the logic of the update property in aip.py is to look for index 0 of the changelog and fall back to the created date if there isn't a changelog.

Perhaps the order of the changelog entries is upside down? Because, the author also updated the changelong in the same change and it still looks like the logic fell back to the created time.

[ryanmcginnis]

This seems important to fix soon. It gives the incorrect impression that the information is out of date.