[ActivityManager] Improve task order of getRunningTasks.

Symptom:
During switching task in same stack, the first result
of getRunningTasks will be the behind stack's top task.
e.g.
 App Task X is starting task Y, the first entry may be home.

Root Cause:
TaskRecord's lastActiveTime is updated when pausing
or resuming. When X task launch a new task Y, Y is
on the top of task history, before X complete pause,
Y's lastActiveTime will be 0 because it is a new task.
Then when comparing the front task with other stack,
other stack will be regarded as the newer one.

Solution:
If the stack is focused stack, give the top task with the last time.

Change-Id: I0adc07608e03d333e0120a0dbc52a0fbbbb12f34

services/core/java/com/android/server/am/ActivityStack.java

@@ -3900,16 +3900,18 @@ final class ActivityStack {
     }
 
     void getTasksLocked(List<RunningTaskInfo> list, int callingUid, boolean allowed) {
+        boolean focusedStack = mStackSupervisor.getFocusedStack() == this;
+        boolean topTask = true;
         for (int taskNdx = mTaskHistory.size() - 1; taskNdx >= 0; --taskNdx) {
             final TaskRecord task = mTaskHistory.get(taskNdx);
+            if (task.getTopActivity() == null) {
+                continue;
+            }
             ActivityRecord r = null;
             ActivityRecord top = null;
             int numActivities = 0;
             int numRunning = 0;
             final ArrayList<ActivityRecord> activities = task.mActivities;
-            if (activities.isEmpty()) {
-                continue;
-            }
             if (!allowed && !task.isHomeTask() && task.effectiveUid != callingUid) {
                 continue;
             }
@@ -3938,14 +3940,18 @@ final class ActivityStack {
             ci.baseActivity = r.intent.getComponent();
             ci.topActivity = top.intent.getComponent();
             ci.lastActiveTime = task.lastActiveTime;
+            if (focusedStack && topTask) {
+                // Give the latest time to ensure foreground task can be sorted
+                // at the first, because lastActiveTime of creating task is 0.
+                ci.lastActiveTime = System.currentTimeMillis();
+                topTask = false;
+            }
 
             if (top.task != null) {
                 ci.description = top.task.lastDescription;
             }
             ci.numActivities = numActivities;
             ci.numRunning = numRunning;
-            //System.out.println(
-            //    "#" + maxNum + ": " + " descr=" + ci.description);
             list.add(ci);
         }
     }