Change AddressPickerDialog to use MultiInviter

[jryans]

We should change AddressPickerDialog to use MultiInviter like all other invite flows. This would allow Riot to have only one code path for initiating invites. In addition, it would change the address picker dialog from "client looks up 3PID" to "client sends 3PID invite to HS for lookup", which is how the other invite flows are functioning today.

[t3chguy]

Address picker is used for non-invite things too though, like adding rooms to communities

[turt2live]

fwiw the reason why we do a lookup is for UX purposes, not for invite purposes. #10556 (comment) has a bit more detail.

Is there anything actionable here? It seems like we want to do the lookup intentionally, so MultiInviter means nothing here.

[jryans]

No, it seems like now that we've talked through the UX, we actively want to keep things as they are here.