amp-lightbox should reset its stacking context when it opens.

We missed enforcing `amp-lightbox` to be a child of `body` like we did `amp-sidebar`, now depending on where in DOM developers add `amp-lightbox` they may run into stacking context issues like https://github.com/ampproject/amphtml/issues/21777 .

We should either move `amp-lightbox` to body before opening or have it reset the stacking context when opened and set it back when closed.

This can be done by adding
```
-amp-lightbox-ancestor {
z-index: auto;
position: static;
opacity: 1;
transform: none;
-webkit-filter: none;
perspective: none;
transform-style: flat;
-webkit-mask: none !important;
transition: none !important;
-webkit-overflow-scrolling: auto;
}
```
to all ancestors.

Start by locating the amp-lightbox implementation and compare its open/close behavior with amp-sidebar, which the issue names as the precedent. Reproduce the stacking-context case from issue 21777, then verify that opening and closing amp-lightbox handles the listed ancestor styles without regressions.

Written by the indexing model from the issue text.