[PiP2] Do not stop PiP-ing activity

Say we have two transitions T1, T2.
T1 is sending a potential PiP candidate TO_BACK
and hence once the visibility of that activity is commited on finish,
it will try and stop the pip activity.

Now say T2 is trying to put that activity into a pinned task. But in
PiP2, Shell is to decide whether to call
WCT#movePipActivityToPinnedRootTask(), which is supposed to remove pip
activity from ATS#mStoppingActivities.

The issue is that if T2 is "requested" first via requestStartTransition
into Shell, the Core is idle to start processing stopping activities,
and we might stop a potential PiP-ing activity briefly.

Solution: make sure to explicitly collect the PiP activity as we are
requesting TRANSIT_PIP and starting the collection process.
This way ATS#processStoppingAndFinishingActivities() will actually skip
stopping it, as it deems it an "animating" activity.

Bug: 390675659
Flag: com.android.wm.shell.enable_pip2
Test: manually tap elsewhere in overview to enter PiP
Test: legacy-enter PiP in gesture nav mode
Change-Id: I21bb6b118dbe2a3c76199739de8a09a1f86c74c8