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/test/src/main/kotlin/io/element/android/libraries/matrix/test/room/FakeMatrixRoom.kt

@@ -34,9 +34,8 @@ import io.element.android.libraries.matrix.test.A_ROOM_ID
 import io.element.android.libraries.matrix.test.A_SESSION_ID
 import io.element.android.libraries.matrix.test.timeline.FakeMatrixTimeline
 import io.element.android.tests.testutils.simulateLongTask
-import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.MutableStateFlow
-import kotlinx.coroutines.flow.emptyFlow
+import kotlinx.coroutines.flow.StateFlow
 import java.io.File
 
 class FakeMatrixRoom(
@@ -126,13 +125,9 @@ class FakeMatrixRoom(
         updateMembersResult
     }
 
-    override fun syncUpdateFlow(): Flow<Long> {
-        return emptyFlow()
-    }
+    override val syncUpdateFlow: StateFlow<Long> = MutableStateFlow(0L)
 
-    override fun timeline(): MatrixTimeline {
-        return matrixTimeline
-    }
+    override val timeline: MatrixTimeline = matrixTimeline
 
     override fun open(): Result<Unit> {
         return Result.success(Unit)