[PiP2] Clean up PiP if enter is aborted

Sometimes enter PiP might actually might actually
get applied on the Core side, make WM state changes, but
the transition might get aborted (e.g. due to screen going to sleep).

In such cases, we might end up with a WM Core <-> Shell inconsistency,
where Shell either gets a no-op transition or a transition without
a pinned task target.

We can try and resolve such inconsistencies on the Shell side by
removing the task that was resolved as the PiP candidate upon
transition request; this effectively cleans up the WM state to match
the Shell state.

Note: repro-ing this issue isn't easy and I could only do it 2/10 times
on average with the animation duration scaled down by 2x on the device.

Bug: 424294158
Flag: com.android.wm.shell.enable_pip2
Test: swipe-pip-to-home with auto-enter and lock the screen quickly
Change-Id: Ie977d6c80b2b7903ac1eee43b22877acc41c5c72