Support for alternate homeserver in the android app

[elinorbgr]

Would be nice to be able to use an other HS than matrix.org, for people who have their own.

Adding a third field, which defaults to "https://matrix.org/' could be an option

[opk12]

A sub-use-case is wanting to join the federation AND have a home non-fed server. The app would be a very lightweight (few MB) workaround for the lack of multiple account support in Element and the other mainstream clients, which are hundreds of MB nowadays.