Added StackId object for checking what features a stack supports

Helps make the code easier to follow since we are no longer checking
multiple stack ids at various decision points.

Bug: 25282299
Change-Id: Ifa6864a1ef56ce2eca4c94f87a4e0b993de987cd
diff --git a/services/core/java/com/android/server/policy/PhoneWindowManager.java b/services/core/java/com/android/server/policy/PhoneWindowManager.java
index b38b9ce..209751e 100644
--- a/services/core/java/com/android/server/policy/PhoneWindowManager.java
+++ b/services/core/java/com/android/server/policy/PhoneWindowManager.java
@@ -16,8 +16,8 @@
 
 package com.android.server.policy;
 
-import static android.app.ActivityManager.DOCKED_STACK_ID;
-import static android.app.ActivityManager.FREEFORM_WORKSPACE_STACK_ID;
+import static android.app.ActivityManager.StackId.DOCKED_STACK_ID;
+import static android.app.ActivityManager.StackId.FREEFORM_WORKSPACE_STACK_ID;
 import static android.view.WindowManager.LayoutParams.*;
 import static android.view.WindowManagerPolicy.WindowManagerFuncs.LID_ABSENT;
 import static android.view.WindowManagerPolicy.WindowManagerFuncs.LID_OPEN;