I would like to know this as well.
From my understanding, the emoji icon and the font is embedded inside the .ttf file (the one inside the hwt/zip file)
I know this because when I compare the ttf file size for the product sans is only 120kb whereas the emoji ios12 is >20mb.
So I guess the only way we can achieve is to find the editor for this emoji pack and put in the product sans font, and generate it into hwt file.
I don't believe you can. Snapchat is using their own set of emojis integrated into the app. You should however see your ios emojis when adding captions to a snap. Note that people who view it will also see the ios emojis - as this is integrated in the picture when being sent. However the emojis within the app itself (stickers, friend emojis, etc...) will be the stock android emojis.