amp-img does not apply class on load when [class] amp binding is defined under mustache template

## What's the issue?

`amp-img` does not apply `class` on load when `[class]` amp binding is defined under `mustache` template

Briefly describe the bug/feature request.

When `class` is defined for the `amp-img` and `[class]` binding is defined, only the `[class]` binding takes effect but not the `class` values on the page load. Also observed if no `[class]` binding is defined the `class` values take effect. In the attached example (given instructions on how to run on mac local using python 3), the class values defined in the `amp-img` ie: `mtop10` and `mtop10 none` do not take effect. However when the buttons are clicked the bound values of `[class]` work as desired.

## How do we reproduce the issue?

```
> download attached zip file on mac - amp-gbug.zip. # attached in the bug (pls. search)
> unzip amp-gbug.zip
> # Follow README.md
```

Also added here for reference

Reproducing steps for bug

### Setup

```
> virtualenv -p python3 amp_env
> source amp_env/bin/activate
> pip3 install -U flask-cors requests
```

### Run

```
> export FLASK_APP=bug.app
> export FLASK_ENV=development
> flask run
```

### Test

Using Browser go to http://127.0.0.1:5000/static/html/bug.html

### Bug Description

You will see following UI on page load (2 images) should be just first image ONLY (as the `class` values have not taken effect).

#### Page Load


Once you click the buttons the code works as expected

#### Click First Button


#### Click Second Button

[](url)

### Tips on Rendering Bugs

Refer attached ZIP file and instructions

## What browsers are affected?

All browsers
Mac

My Chrome Version - Version 89.0.4389.90 (Official Build) (x86_64)
My Safari Version - Version 14.0.3 (15610.4.3.1.7, 15610)

## Which AMP version is affected?

Not sure if it was always broken for mustache
AMP version

```






```

Attached ZIP file:
[amp-gbug.zip](https://github.com/ampproject/amphtml/files/6174996/amp-gbug.zip)

Start with the attached ZIP's README and static/html/bug.html, then reproduce the initial page-load behavior using the listed AMP scripts and local Flask setup. Trace how amp-img combines its static class with the [class] binding inside the mustache template. Done means the initial static classes apply on load while subsequent bound class changes still work.

Written by the indexing model from the issue text.