Fix back stack for PiP from multi-activity Task

Details in design doc go/pip-and-back

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/g8lbXnC8ln0t54ARxfPELo
Bug: 184758170
Test: ensure back stack works for Netflix, see video
Test: atest PinnedStackTests
Test: atest RootWindowContainerTests
Change-Id: I9e2f8d0d50dd107ff30fd7afe6273d8347aac803