Fix Keyboard covered input box after leaving split screen

When leaving split-screen, task organizer will trigger transaction to
reparent task and then invoking updateAboveInsetsState for all windows
when updating the IME layering target through the below call path:

InsetsStateController.updateAboveInsetsState
   DisplayContent.setImeLayeringTargetInner
     DisplayContent.computeImeTarget
       DisplayContent.updateFocusedWindowLocked
         ..
           DisplayContent.layoutAndAssignWindowLayersIfNeeded
             Task.reparent
               ..
                WindowOrganizerController.applyTransaction

However, DC#forAllWindows will skip IME windows in
WindowState#applyImeWindowsIfNeeded since split-screen mode not yet be
changed. that caused the application will get mAboveInsetsState when
dispatching to the client because system thought the app window is above
IME.

Use isSplitScreenModeActivated() to consolide the split-screen check in
case misjudge for this issue.

Fix: 186746922
Test: atest InsetsStateControllerTest
Test: manual as issue steps
Change-Id: I47bcb1234a49ea35c5e0d7281973fb3237e420eb