Fix App Engine's expiration calculation

[theacodes]

Resolves googleapis/google-cloud-python#3937

[dhermes]

LGTM, though WDYT about adding a script here that will deploy a new GAE app and run unit tests there (with results available via curl, e.g.)

[theacodes]

LGTM, though WDYT about adding a script here that will deploy a new GAE app and run unit tests there (with results available via curl, e.g.)

We have one in system_tests, it just doesn't do any introspection on the token or expiry - it just makes an authenticated request and verifies it works.