Support multiple tasks per split screen in SplitBounds

- Introduce `leftTopTaskIds` and `rightBottomTaskIds` lists to `SplitBounds`.
- Populate these lists by retrieving the relevant task IDs from Stage Tasks.

Bug: 346295292
Bug: 360782849
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: atest WMShellUnitTests

Change-Id: Ia644f831ae13618b10a275b76aeca4b0bc19cb46