Do not letterbox the activity in PiP mode

There are chances that a Letterbox is added to the Task entering PiP
mode, simply ignore such request.

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/dPygRdWjyGltlCmQU0TPZn
Bug: 283008030
Test: Follow the reproduce steps in bug, see video
Change-Id: I83c5d32ef7cff4b3b6c85a698d068935c83c9423