Put PIP windows above transition animations.

PIP activities are already on top of everything else, but during
animation we increase the layer of the animated window for the duration
of the animation and it may cover the PIP. By forcing the same
adjustment on PIP windows we will keep them above animating windows.

Bug: 26015827
Change-Id: I8f7a87f41fed24b3e520fb599a94cf24cc2eeb50
diff --git a/services/core/java/com/android/server/wm/DisplayContent.java b/services/core/java/com/android/server/wm/DisplayContent.java
index 4bbf586..898a9a4 100644
--- a/services/core/java/com/android/server/wm/DisplayContent.java
+++ b/services/core/java/com/android/server/wm/DisplayContent.java
@@ -237,8 +237,8 @@
     }
 
     void moveStack(TaskStack stack, boolean toTop) {
-        if (stack.mStackId == PINNED_STACK_ID && !toTop) {
-            // Pinned stack is always-on-top silly...
+        if (StackId.isAlwaysOnTop(stack.mStackId) && !toTop) {
+            // This stack is always-on-top silly...
             Slog.w(TAG, "Ignoring move of always-on-top stack=" + stack + " to bottom");
             return;
         }