Defer onTaskInfoChanged while transitioning to PiP

The ApiDemos app triggers onTaskInfoChanged when transitioning from
landscape to portrait PiP due to its implementation (fixed in a separate
CL in same topic). Here we defer applying the TaskInfo received in
onTaskInfoChanged till the entering transition finishes.

Video: http://rcll/aaaaaabFQoRHlzixHdtY/c1n3cqwBydRxDrwxKHqI0H
Bug: 180630127
Test: enter PiP w/ autoEnter from landscape in button navigation
Test: atest WMShellUnitTests:com.android.wm.shell.pip.PipTaskOrganizerTest
Change-Id: I3bc07dbb45a281d5c9b0bbfff7478e57b8944c60