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)`)

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/room/MatrixRoom.kt

@@ -58,9 +58,9 @@ interface MatrixRoom : Closeable {
      */
     suspend fun updateMembers(): Result<Unit>
 
-    fun syncUpdateFlow(): Flow<Long>
+    val syncUpdateFlow: StateFlow<Long>
 
-    fun timeline(): MatrixTimeline
+    val timeline: MatrixTimeline
 
     fun open(): Result<Unit>