Lightbox controls do not appear when in a touch environment (mouseless)

### Description

The `` component should create Prev/Next buttons on the left and right side of the screen.
When emulating a phone (or on an actual phone), the controls never appear.

See:

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I believe the root cause is the addition of `i-amphtml-screen-reader`:

https://github.com/ampproject/amphtml/blob/4b32cefe4bd761d27754db5c9bac76cad6a1b199/extensions/amp-lightbox-gallery/0.1/lightbox-controls.js#L64-L72

### Reproduction Steps

1. In Chrome, enable mobile emulation
1. Go to https://playground.amp.dev/?url=https%3A%2F%2Fpreview.amp.dev%2Fdocumentation%2Fexamples%2Fcomponents%2Famp-lightbox-gallery&format=websites
2. Click any image and notice that the left/right arrows do not appear

@ampproject/wg-components

Start with extensions/amp-lightbox-gallery/0.1/lightbox-controls.js, especially lines 64–72 where i-amphtml-screen-reader was added. Reproduce the issue using Chrome mobile emulation and the linked AMP Playground example, then verify that amp-lightbox-gallery shows left and right controls in a touch environment without hiding them from sighted users.

Written by the indexing model from the issue text.