Fix multi-activity pip flickers

The flicker on enter is a simple fix, the original pip task
wasn't collected so it's surface hide wasn't part of the sync
transaction. Just collect it to solve that.

For disappearing in overview, the problem is more subtle:
With shell-transitions, pip-entry happens at the end of
the transient-launch transition. This means the pipping activity is
starting off behind the home task. With multi-activity, a new task
is created to host the pip, but it is placed on TOP, so when we
reparent the pip activity into it, because it is going from
visibleRequested=false to visReq=true, it triggers focus logic
with in-turn triggers RecentTasks logic. Because it is a temporary
task, RecentTasks gets confused and marks the pip's original task
as hidden. This then triggers removal on idle which in-turn removes
the pip activity since it's really part of the original task.

To fix this, we set the windowing-mode of the new (temporary) task
to PINNED *before* reparenting the activity into it. That way it
doesn't try to grab focus.

Also, to make things clearer, this re-arranges some things:
multi-activity pip-task doesn't have fragments, so moved the
fragment code into the singleActivity case so its more obvious
that this is the case. Did the same thing with "reparent to
activity TDA" because the new task is created there already.

Also made the temporary task match the original task's override
windowing mode (instead of UNDEFINED), otherwise it can create
flickers when pipping from a task in a different mode from the
display.

Bug: 269910373
Test: open multi-activity pip app, then swipe to home.
Change-Id: I9e1b36b47e7de331db2fcea3d1b1b6cfd2d34a26