[web] Hide that InputState.sendTextMessage returns a Promise

Summary:
While working on the unused-promise Flow lint, I noticed that sendTextMessage doesn't actually await the call to sendTextMessageAction. This makes it dangerous to expose as an API to callers, who probably assume the keyserver call is complete when the Promise resolves.

I could make it await here, but since apparently no callsites need a Promise currently, I decided to just make the API return a mixed here for now.

Depends on D10161

Test Plan: Flow

Reviewers: atul, bartek, michal

Reviewed By: michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10162