Read config.xml file from Riot web client

[bmarty]

When the user enters a homeserver url, he can actually enter a Riot web client url, and RiotX will retrieve the default homeserver from the config file.

But the work is not complete.

Riot-Web impl: https://github.com/vector-im/riot-web/blob/eac86b60536cbcf421f90bff30e1d21cd0a90c00/src/vector/app.tsx#L184-L236

in summary, choose first applicable of:

use `default_hs_url` [& `default_is_url`] if available
use `default_server_config` if available
use `default_server_name` if available

For the moment, RiotX only looks at default_hs_url, so it does not work for instance for https://riot.im/app

Code: https://github.com/vector-im/riotX-android/blob/5f540a5b45dd1fe32354ece35c2c00a3bafe6aa0/matrix-sdk-android/src/main/java/im/vector/matrix/android/internal/auth/data/RiotConfig.kt#L27

Ref: https://github.com/vector-im/riot-web/blob/develop/docs/config.md

Also another thing to consider is that Riot web reads two config.json files config.$domain.json if exists, otherwise config.json (where $domain is the riot domain e.g chat.mozilla.org)