Polish ActivityEmbedding enter/exit PiP (1/n)

1. When moveActivityToPinnedRootTask with creating a new Task for PiP,
   make sure the Task's initial bounds is the same as the activity
   parent TaskFragment so the animation starts from the correct bounds.
2. When exit PiP to previous Task, make sure we are animating the
   correct window surface. For the previous implementation. there can
   also be TRANSIT_CHANGE change for entering ActivityEmbedding split
   (from PiP) in the same transition.

Bug: 207070762
Test: atest WmTests:RootWindowContainerTests
Test: atest WmTests:TransitionTests
Change-Id: Ifba090ad9ac9fb7033d343eab1c87c1a67bb9c11