Use new install referrer API to configure homeserver url

Install referrer is used in Riot-Android to pre-fill homeserver (and identity server).

This is not done yet in RiotX, but it's a TODO in the code.

So we can use the new API in RiotX. That said, we have to think how this can be integrated in the new signin/signup flow.

_Mail received from Google_:

Hello Google Play Developer,

We recently [announced](https://android-developers.googleblog.com/2019/11/still-using-installbroadcast-switch-to.html) that we’ll be deprecating the install_referrer intent broadcast mechanism. Because one or more of your apps uses this intent to track referrals, we wanted to ensure you make the switch before March 1, 2020. After this date, new versions of the Play Store app will no longer broadcast the install_referrer intent after app installs.

Action required

[Migrate to the Play Install Referrer API](https://developer.android.com/google/play/installreferrer/library.html) to track your app installs for the following apps and/or games.

Affected apps

im.vector.app
im.vector.alpha
The Play Install Referrer API offers better performance, uses a secure communication channel between your app and the Play Store, and offers a more robust solution against spoof and attribution fraud.

We know this change may require some additional work on your part and appreciate your understanding.

Thank you for being a part of Google Play.