Allow other PiP changes in remote-and-PiP case

It is possible that a transition with a remote registered
ends up collecting a change involving a PiP task, for example
due to that PiP activity finishing; and these cases do not
have to be limited to enter-PiP type changes (like upon
notification tap to enter PiP).

So instead, generalize openIntentWithRemoteAndPipOrDesktop
type of mixed transition to pluck out other types of PiP changes
when PiP2 is enabled and route them through mPipHandler#startAnimation()
which will itself choose a proper animation for the PiP part of the
transition.

Also added a new DefaultMixedTransitionTest unit test to check for
PiP + remote cases being split properly as a mixed transition.

Bug: 442588134
Flag: EXEMPT bugfix
Test: atest WMShellUnitTests:DefaultMixedTransitionTest

Change-Id: I9349cfae64293b61d3783d1223545b91f4c0fc10