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)`)

features/roomdetails/impl/src/main/kotlin/io/element/android/features/roomdetails/impl/edit/RoomDetailsEditPresenter.kt

@@ -52,7 +52,7 @@ class RoomDetailsEditPresenter @Inject constructor(
 
     @Composable
     override fun present(): RoomDetailsEditState {
-        val roomSyncUpdateFlow = room.syncUpdateFlow().collectAsState(0L)
+        val roomSyncUpdateFlow = room.syncUpdateFlow.collectAsState()
 
         // Since there is no way to obtain the new avatar uri after uploading a new avatar,
         // just erase the local value when the room field has changed