Ignore resize request when exiting PiP

When PipTaskOrganizer#removePip is called, the animator in
PipMotionHelper may still be running and issues a conflict window
container transaction at its end. This causes the activity resuming to a
weird bounds after drag to dismiss and re-open, in split-screen mode.

Fixes this by
- In PipMotionHelper, if it's a dismiss action, do not issue the window
container transaction
- In PipTaskOrganizer, if we know we're exiting, do not accept any
resize request

Video: http://rcll/aaaaaabFQoRHlzixHdtY/fCOxvnTXu9RtFYERIdskNf
Bug: 159943928
Test: see video
Change-Id: Ia9dd02f9707f9da74166bee24a015c2bb4e223c2