These widgets have different logic in:
WP_Widget_Media_Video::render()
So override the value of wp_video_shortcode(), which renders these.
Produce 'youtube' or 'vimeo' shortcodes.
And this plugin's handlers will create <amp-youtube> and <amp-vimeo>.
Another plugin is also free to override those shortcodes.

Similarly to how the other widget overrides work.
Also, adjust tests for this.

This will allow another plugin to override this.
Inspired by the Jetpack plugin's return of the initial value.

Thanks to Weston for mentioning how we could use 'post_gallery.'
Using this removes the need to subclass that widget.
The widget calls gallery_shortcode(), which has the filter:
'post_gallery'
As Weston mentioned, Jetpack filters 'post_gallery'.

These are now in the embed classes.
Also, create test classes for them.

When they use 'Media Library' videos, they output <amp-video>.
The AMP_Video_Sanitizer added a 'layout' value of 'fixed-height'.
The height of 400px didn't look good, as the aspect ratio was wrong.
Instead, make video widgets that use <amp-video> use layout=responsive.
Detect this by checking that neight 'width' or 'height' are present.
inject_video_max_width_style() removes these attributes for the widget.
Otherwise, retain the previous logic.

This reverts commit a330342.

Props @westonruter for the details of how to do this.
Overrides the widget WP_Widget_Media_Video,
in order to override the callback that strips the height and width.
Also, add a style of 'width:100%' to the <amp-video>.
This ensures that the widget doesn't overflow the container.

Override the 'Text' widget.
Prevent the stripping of <video> width and height attributes.
Also, move the addition of the 'style' attribute.
Before, it wasn't added.