Fix home activity and user switch interactions.

- Make sure Home activity goes in the correct task and on the correct
stack.
- Do not allow different users to be in the same task.
- Do not set stacks aside for each user.

Fixes bug 9775492.

Change-Id: I0e7954e917aac8482a1015a36923e02914e2b692
diff --git a/services/java/com/android/server/wm/Task.java b/services/java/com/android/server/wm/Task.java
index 88eb96e..d9acbb9 100644
--- a/services/java/com/android/server/wm/Task.java
+++ b/services/java/com/android/server/wm/Task.java
@@ -21,11 +21,13 @@
     TaskStack mStack;
     final AppTokenList mAppTokens = new AppTokenList();
     final int taskId;
+    final int mUserId;
 
-    Task(AppWindowToken wtoken, TaskStack stack) {
+    Task(AppWindowToken wtoken, TaskStack stack, int userId) {
         taskId = wtoken.groupId;
         mAppTokens.add(wtoken);
         mStack = stack;
+        mUserId = userId;
     }
 
     DisplayContent getDisplayContent() {