Fix DocumentsUI unable to launch

The root activity of the task was finished while reset tasks.

Bug: 147223901
Test: atest IntentTests
Change-Id: I1a72b741096fec01f46220ef4420207e529e05be
diff --git a/services/core/java/com/android/server/wm/ResetTargetTaskHelper.java b/services/core/java/com/android/server/wm/ResetTargetTaskHelper.java
index a5c90a1..e310fc1 100644
--- a/services/core/java/com/android/server/wm/ResetTargetTaskHelper.java
+++ b/services/core/java/com/android/server/wm/ResetTargetTaskHelper.java
@@ -91,6 +91,9 @@
     }
 
     private boolean processActivity(ActivityRecord r, boolean isTargetTask) {
+        // End processing if we have reached the root.
+        if (r == mRoot) return true;
+
         mAllActivities.add(r);
         final int flags = r.info.flags;
         final boolean finishOnTaskLaunch =