amp-subscription + amd-ad: subscriptions-display ist ignored and hidden ad is loaded

## What's the issue?

When using `subscriptions-display` attribute to hide ads for subscribed users results, ads are loaded in a hidden iframe.

While investigating the problem I noticed this isn't always the case. It seems this only affects `amp-ads` configured with `layout="fluid"` (for multi-size ads). However, `amp-ads` configured with `layout="fixed"` are behaving as expected.

## How do we reproduce the issue?

You'll need an amp page with `amp-subscription` to reproduce the error. Try to hide/show ads based based on subscription state and use the following code snippets.

**Works as expected:**

```

**Doesn't work:**

```

The latter one results in a hidden div, which loads the ad (see the iframe within the `amp-ad` container).

Reproduce the issue with the provided amp-subscription and amp-ad snippets, comparing fluid and fixed layouts while toggling the subscription state. Trace why a fluid amp-ad inside subscriptions-display loads an iframe while hidden, and consider the issue complete when hidden fluid ads no longer load while the fixed-layout behavior remains correct.

Written by the indexing model from the issue text.