Fix caching IME status for split screen even when it is not activated

Add back the divider bar view's visibility checks which was removed by
ag/14318188 in order to prevent caching IME status while the split is
not activated.

Fix: 188807172
Test: lock and unlock the device with password keyguard while split
activated, observed the split divider handler is visible.
Test: rotate the device while showing IME, then dock two apps to enter
split mode, observed the split divider handler is visible.

Change-Id: Iaa4a68f98bab6a832f4b2cbd448cb42b1ecb9bbb

libs/WindowManager/Shell/src/com/android/wm/shell/legacysplitscreen/DividerView.java

@@ -408,7 +408,7 @@ public class DividerView extends FrameLayout implements OnTouchListener,
     }
 
     boolean isHidden() {
-        return mSurfaceHidden;
+        return getVisibility() != View.VISIBLE || mSurfaceHidden;
     }
 
     /** Starts dragging the divider bar. */