Return all visible activities for VIS

Before, we only return the top activities of each visible root Task
(with handle split screen specially), which doesn't cover:
1. Desktop Windowing, which can have multiple leaf Tasks in one root.
2. Activity Embedding, which can have multiple leaf TaskFragments in one
   leaf Task.
3. Paused activities behind another translucent activity.

Now, we return all visible activities.

Bug: 434114340
Flag: com.android.window.flags.return_all_visible_activities_for_vis
Test: atest WmTests:RootWindowContainerTests
Change-Id: Ib07febfc65ed800c7ecbcd7086a58dffdd771798