Start refactoring some business logic into view models

As Element Call grows in complexity, it has become a pain point that our business logic remains so tightly coupled to the UI code. In particular, this has made testing difficult, and the complex semantics of React hooks are not a great match for arbitrary business logic. Here, I show the beginnings of what it would look like for us to adopt the MVVM pattern. I've created a CallViewModel and TileViewModel that expose their state to the UI as rxjs Observables, as well as a couple of helper functions for consuming view models in React code.

This should contain no user-visible changes, but we need to watch out for regressions particularly around focus switching and promotion of speakers, because this was the logic I chose to refactor first.

src/livekit/MediaDevicesContext.tsx

@@ -26,7 +26,6 @@ import {
 } from "react";
 import { createMediaDeviceObserver } from "@livekit/components-core";
 import { Observable } from "rxjs";
-import { logger } from "matrix-js-sdk/src/logger";
 
 import {
   isFirefox,
@@ -83,14 +82,7 @@ function useMediaDevice(
   // Tragically, the only way to get device names out of LiveKit is to specify a
   // kind, which then results in multiple permissions requests.
   const deviceObserver = useMemo(
-    () =>
-      createMediaDeviceObserver(
-        kind,
-        () => {
-          logger.error("Error creating MediaDeviceObserver");
-        },
-        requestPermissions,
-      ),
+    () => createMediaDeviceObserver(kind, requestPermissions),
     [kind, requestPermissions],
   );
   const available = useObservableState(deviceObserver, []);