Update "Building the Source Code" guide

The guide on how to build Briar from source is a bit outdated. It should be updated for the latest version of Android Studio.

added Document Documentation S: in progress T: feature labels

@nicoalt Should I create a page like this for Briar GTK on the website? Or will you rather keep the information in the Readme?

@c1e0 I think I'll keep the information in the readme, since I most likely would forget about updating that page in parallel However, thanks for asking!

mentioned in merge request !48 (merged)

mentioned in commit 806d923a

closed with merge request !48 (merged)