Mark pinned activity task as auto-removeable from recents.

Tasks resumed from Picture-in-Picture (PIP) mode originating from a
non-single activity task were unintentionally added to the recents.
We initially observed this issue on the 24D1-dev branch when exiting
PIP (b/323315206), which was fixed by ag/26565884 on the main branch.

However, we encountered another scenario when exiting PIP that
originated from a non-single activity task with split pair. It
involves reorder operations (`mHierarchyOps`) to split paired in
recents. The pinned activity task was being added to the recents
while resuming the top PIP activity.

This change takes a different approach compared to previous attempt.
Instead of checking if the task is a pinned activity task from a
non-single activity task for each `RecentTasks.add`, this change only
marks the pinned activity task as auto-removable and then lets
`reparent` in `Task.setWindowingMode` cleanup resources for destory.
This approach prevents empty pinned task from appearing in recents.

Test: atest WmTests:TaskTests WmTests:RootWindowContainerTests
Fix: 348085976
Flag: EXEMPT bugfix
Change-Id: I8ed5938f122988383b11fe0820723de980313540