MatrixRoom API refinement (#719)

- `syncUpdateFlow` becomes a `val` and always returns the same instance of the underlying `StateFlow` instead of different `Flow` instances to allow consumers not to remember the `Flow` and not to specify an unneeded initial value.
- `timeline` becomes a `val` as it already always returns the same instance.
- Amends calling code accordingly
- Removes a few unneeded `val`s in `RustMatrixClient
- Fixes a small bug in `MessagesPresenter` that allowed to sometime show a newly created room's name as "Empty room" (changes `LaunchedEffect(syncUpdateFlow)` to `LaunchedEffect(syncUpdateFlow.value)`)

samples/minimal/src/main/kotlin/io/element/android/samples/minimal/RoomListScreen.kt

@@ -82,7 +82,7 @@ class RoomListScreen(
                 withContext(coroutineDispatchers.io) {
                     matrixClient.getRoom(roomId)!!.use { room ->
                         room.open()
-                        val timeline = room.timeline()
+                        val timeline = room.timeline
                         timeline.apply {
                             // TODO This doesn't work reliably as initialize is asynchronous, and the timeline can't be used until it's finished
                             paginateBackwards(20, 50)