Fix IME aboves popupwindow when the app is in split-screen

As now the new split-screen system has been migated to wmShell
with using WINDOWING_MODE_MULTI_WINDOW to setup the split task.
so inSplitScreenWindowingMode() will no longer be true since this
method is used for legacy split-screen.

It affects WindowState#needsRelativeLayeringToIme can not
check if the app is in split-screen window and makes system can't
set relative layering to IME for popupWindow to make it above IME.

Note that setting relative layering with IME also benefits for other
non-fullscreen windowing mode like in bubble/freeform mode, the
dropdown menu can above the IME when the parent window is IME
layering target.

Changing to use DC#isImeAttachedToApp() to fix this layering issue.

Fix: 201504527
Test: atest ZOrderingTest#\
 testPopupWindowAndParentIsImeTarget_expectHigherThanIme_inMultiWindow
Test: manual as issue steps
   1) Go to "Create contact" in Phone.
   2) Enter split-screen mode.
   3) Click on a text box of "Create contact".
   4) Click the menu button to show the menu of "Create contact".
   5) Expect the layer of the menu should above the IME surface.

Change-Id: I2f466a2305e56e4fd2a0c509d7c8027104cb51b6