Fixed bug with pop-up window placement at the bottom in split-screen

WindowState.mInsetFrame is used to represent a temp. offset of a task
due to something like docked stack resizing or IME currently on screen.
In the case of pop-up windows (or any child window that wants to layout
in the parent frame) we were incorrectly setting the containing frame
because we were checking to see if the mInsetFrame wasn't empty. This
check isn't needed for child windows that want to layout in parent frame
because the parent frame will already have the mInsetFrame applied to
its frame which will automatically trickle down to the display and content
frame it forwards to its child windows to use.

Bug: 28389714
Change-Id: If89d40845a5a14aa60abcdedef2385b1fe7bfee3