RoomListViewModel: Track the index of the active room in the list (#29519)

* Introduce a hook to track active room

This hook simply keeps a state which tracks the index of the active room
in the list of rooms passed through props. This index will be recomputed
if the active rooms changes or if the list itself changed.

* Use hook in the view model

* Write tests

* Fix broken tests

test/unit-tests/components/views/rooms/RoomListPanel/EmptyRoomList-test.tsx

@@ -31,6 +31,7 @@ describe("<EmptyRoomList />", () => {
             canCreateRoom: true,
             shouldShowMessagePreview: false,
             toggleMessagePreview: jest.fn(),
+            activeIndex: undefined,
         };
     });
 

test/unit-tests/components/views/rooms/RoomListPanel/RoomList-test.tsx

@@ -40,6 +40,7 @@ describe("<RoomList />", () => {
             createRoom: jest.fn(),
             createChatRoom: jest.fn(),
             canCreateRoom: true,
+            activeIndex: undefined,
         };
 
         // Needed to render a room list cell

test/unit-tests/components/views/rooms/RoomListPanel/RoomListPrimaryFilters-test.tsx

@@ -34,6 +34,7 @@ describe("<RoomListPrimaryFilters />", () => {
             activeSortOption: SortOption.Activity,
             shouldShowMessagePreview: false,
             toggleMessagePreview: jest.fn(),
+            activeIndex: undefined,
         };
     });
 

test/unit-tests/components/views/rooms/RoomListPanel/RoomListView-test.tsx

@@ -35,6 +35,7 @@ describe("<RoomListView />", () => {
         canCreateRoom: true,
         toggleMessagePreview: jest.fn(),
         shouldShowMessagePreview: false,
+        activeIndex: undefined,
     };
     const matrixClient = stubClient();