Use vanniktech's library for emojis

So I was working on updating to the latest emojis in spritegen!1 (closed) but did not have success generating updated sprites. The process is currently quite complicated involving several dependencies and many steps in order to generate the emoji sprites.

In #940 (closed) I mentioned there's a library by vanniktech which serves exactly the functionality Briar wants to have:

A simple library to add Emoji support to your Android app. In a PopupWindow Emojis can be chosen.

One can choose from four different emoji providers, where Briar likely wants to use Google's one (#940 (comment 28412)):

It's true that we can't provide the same icons for all users, but we can reduce the number of users who see different icons by using the latest Android icons.

This issue is for discussing and implementing the switch from Briar's current way of implementing emojis to vanniktech's library. Interesting for this task are all commits with 'emoji' in their name.