Check task switch by comparing leaf task.

In multi-window mode, the previous top root task can be the parent of
the task of launched activity, so the task switch should comparing by
leaf task level.

Bug: 231363211
Test: atest ActivityStarterTests
Test: enter split screen, start another activity from an existing task,
verify there won't create starting window on that task.

Change-Id: Iaf59e405671a1b8a6674bab4f49ee36b6eb24da3