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/KeyguardController.java b/services/core/java/com/android/server/am/KeyguardController.java
index e03c530..7101fc4 100644
--- a/services/core/java/com/android/server/am/KeyguardController.java
+++ b/services/core/java/com/android/server/am/KeyguardController.java
@@ -342,8 +342,12 @@
             // show on top of the lock screen. In this can we want to dismiss the docked
             // stack since it will be complicated/risky to try to put the activity on top
             // of the lock screen in the right fullscreen configuration.
-            mStackSupervisor.moveTasksToFullscreenStackLocked(DOCKED_STACK_ID,
-                    mStackSupervisor.mFocusedStack.getStackId() == DOCKED_STACK_ID);
+            final ActivityStack stack = mStackSupervisor.getStack(DOCKED_STACK_ID);
+            if (stack == null) {
+                return;
+            }
+            mStackSupervisor.moveTasksToFullscreenStackLocked(stack,
+                    mStackSupervisor.mFocusedStack == stack);
         }
     }