Remove Task#numFullscreen & ActivityRecord#sendingToBottom/remove (70/n)

- Task#numberFullscreen was only used in one location where we
needed to figure-out if the task has occluding activities.
Just use the hierarchy to figure that out vs. keeping track of
a variable that adds confusion.
- ActivityRecord#sendingToBottom was used to prevent focus and
orientation changes from activity moved to the bottom, however
since we are traversing the hierarchy from top-to-bottom we
should process activities on-top first vs the onces that have
already moved to the bottom.
- ActivityRecord#remove is no longer needed as the object won't
be connected to the hierarchy when it is removed.

Test: They pass!
Bug: 80414790
Bug: 35699615
Change-Id: I23f6501a65bf745dc0fb819c30543cd21241fa91