Allow delete empty TaskFragment in PiP

We ignore WCT for TaskFragment in PiP, but in case the empty
TaskFragment is not removed before the Task enters PiP, we should allow
organizer to remove it.

Also make sure we have all requests in one WCT during
onTaskFragmentInfoChanged.

Bug: 225371112
Test: atest WmTests:TaskFragmentOrganizerControllerTest
Test: atest WMJetpackUnitTests:SplitControllerTest
Test: atest WMJetpackUnitTests:TaskFragmentContainerTest
Change-Id: I718bbb7f6c18c35048c671ed3c131eaf536cf265