amp-timeago causes layout shift

## What's the issue?
amp-timeago causes layout shift, particularly when no default value is given, due to changing width of the element

## How do we reproduce the issue?
Use amp-timeago with layout="fixed-height"

Example:
https://www.livescience.com/amp/21312-freedom-of-the-press.html

## What browsers are affected?
all browsers

## Which AMP version is affected?
2003031842100

Start by reproducing the issue with amp-timeago using layout="fixed-height" and the linked LiveScience example, then inspect the amp-timeago component implementation. Done means the element no longer causes layout shift when its value changes, including when no default value is provided, across the affected browsers.

Written by the indexing model from the issue text.