[native] Create a component for the media gallery on native

Summary:
Here, we create the native component for the thread media gallery. Currently, mediaInfos is defaulted to an empty array, but it will be replaced to hold the media data in subsequent diffs. galleryItemGap refers to the gap between media, and galleryFirstItemGap refers to the first media in each row. galleryItemWidth is determined using Dimensions so we can hopefully ensure that each item takes 1/3 of the width of the gallery.

Encompassing the media in <TouchableOpacity> will be changed in later diffs, alongside just rendering an <Image> component.

Depends on D6465

Test Plan: This will be tested in the next diff, and a screenshot of what the media gallery will look like with sample, hard coded images will be shown.

Reviewers: atul, ginsu, kamil, tomek, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6466