Don't assume there will be a running task

- During tests, if apps and launcher are killed, then there may be no
  running task

Change-Id: Iaef1c4ad0b31cec3c8da7d868e3e7b49d3b09faa

quickstep/src/com/android/quickstep/OverviewCommandHelper.java

@@ -227,7 +227,7 @@ public class OverviewCommandHelper {
         public RecentsActivityCommand() {
             mHelper = getActivityControlHelper();
             mCreateTime = SystemClock.elapsedRealtime();
-            mRunningTaskId = mAM.getRunningTask().id;
+            mRunningTaskId = RecentsModel.getRunningTaskId();
 
             // Preload the plan
             mRecentsModel.getTasks(null);

quickstep/src/com/android/quickstep/RecentsModel.java

@@ -18,6 +18,7 @@ package com.android.quickstep;
 import static com.android.quickstep.TaskUtils.checkCurrentOrManagedUserId;
 
 import android.annotation.TargetApi;
+import android.app.ActivityManager;
 import android.content.ComponentCallbacks2;
 import android.content.Context;
 import android.os.Build;
@@ -94,6 +95,15 @@ public class RecentsModel extends TaskStackChangeListener {
         return mTaskList.getTasks(-1, false /* loadKeysOnly */, callback);
     }
 
+    /**
+     * @return The task id of the running task, or -1 if there is no current running task.
+     */
+    public static int getRunningTaskId() {
+        ActivityManager.RunningTaskInfo runningTask =
+                ActivityManagerWrapper.getInstance().getRunningTask();
+        return runningTask != null ? runningTask.id : -1;
+    }
+
     /**
      * @return Whether the provided {@param changeId} is the latest recent tasks list id.
      */
@@ -134,7 +144,7 @@ public class RecentsModel extends TaskStackChangeListener {
 
         // Keep the cache up to date with the latest thumbnails
         mTaskList.getTasks(mThumbnailCache.getCacheSize(), true /* keysOnly */, (tasks) -> {
-            int runningTaskId = ActivityManagerWrapper.getInstance().getRunningTask().id;
+            int runningTaskId = RecentsModel.getRunningTaskId();
             for (Task task : tasks) {
                 if (task.key.id == runningTaskId) {
                     // Skip the running task, it's not going to have an up-to-date snapshot by the