Moved ActivityDisplay to its own class file.

Clean-up before the next windowing mode refactor.
Also:
- Change the return type of ActivityStack.shouldBeVisible from int
to boolean since it is now binary.
- Removed ActivityStack.mRecentTasks. Changed call points to use
the reference in ActivityStackSuperior.
- Removed ActivityStack.mStacks and ActivityStackSuperior.mStack.
Changed call points to use reference in ActivityDisplay.
- Removed a few unused methods and variables.

Fixes: 66452053
Test: Existing tests pass.
Change-Id: Id9f4c668cdfc638820ad1777db85ac4d2619b85a
diff --git a/services/core/java/com/android/server/am/PinnedActivityStack.java b/services/core/java/com/android/server/am/PinnedActivityStack.java
index a601ee1..33f5664 100644
--- a/services/core/java/com/android/server/am/PinnedActivityStack.java
+++ b/services/core/java/com/android/server/am/PinnedActivityStack.java
@@ -32,9 +32,9 @@
 class PinnedActivityStack extends ActivityStack<PinnedStackWindowController>
         implements PinnedStackWindowListener {
 
-    PinnedActivityStack(ActivityStackSupervisor.ActivityDisplay display, int stackId,
-            ActivityStackSupervisor supervisor, RecentTasks recentTasks, boolean onTop) {
-        super(display, stackId, supervisor, recentTasks, onTop);
+    PinnedActivityStack(ActivityDisplay display, int stackId, ActivityStackSupervisor supervisor,
+            boolean onTop) {
+        super(display, stackId, supervisor, onTop);
     }
 
     @Override