[native] Don't crash yarn cleaninstall if ios/android folders are missing

Summary:
We don't want to have to copy these folders into the keyserver Docker image if we don't have to.

Depends on D3710

Test Plan: Make sure I can build the Node.js keyserver from Docker

Reviewers: palys-swm, jimpo

Reviewed By: palys-swm

Subscribers: Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3712