GFP cookie expiration uses seconds instead of milliseconds

### Description

GFP cookie expiration times are set based on a value derived from an http response header from ad responses. The http header sets the expiration in seconds, but client code in AMP expects milliseconds. This results in expiration dates in the past, and hence no cookies are being set.

### Reproduction Steps

View any AMP page with an A4A ad. See the amp-ff-set-cookies header value and note the "expires" value. Observe that if this value is passed to `new Date(expires).toUTCString()` the date will be in 1970. Observe further that no cookie set in the browser. Multiplying this value by 1000 (converting to ms) makes the date correct.

### Relevant Logs

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### Browser(s) Affected

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### OS(s) Affected

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### Device(s) Affected

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### AMP Version Affected

_No response_