Support fallback attribute in amp-img for amp-carousel 0.2

When i use amp-carousel-0.2.js and "fallback" attribute in amp-img i have error:
**Found more than one images or none in slide!**

**Source code:**

```

```

When using amp-carousel-0.1, this error does not appear.

Start by reproducing the supplied markup with amp-carousel-0.2.js, then inspect how amp-carousel-0.2.js counts amp-img elements in each slide and compare that behavior with amp-carousel-0.1. Done means the nested fallback amp-img is accepted without the “Found more than one images or none in slide!” error.

Written by the indexing model from the issue text.