Get TaskRecord children with getChildAt & co. APIs (54/n)

Helps reduce work with Task level unification.
Bug: 80414790
Test: Existing tests pass

Change-Id: I5e019c7d1f3ed50c4efd628b68fee6990841904d
diff --git a/services/core/java/com/android/server/wm/ActivityMetricsLogger.java b/services/core/java/com/android/server/wm/ActivityMetricsLogger.java
index a0a2967..ef8c020 100644
--- a/services/core/java/com/android/server/wm/ActivityMetricsLogger.java
+++ b/services/core/java/com/android/server/wm/ActivityMetricsLogger.java
@@ -527,8 +527,8 @@
     }
 
     private boolean hasVisibleNonFinishingActivity(TaskRecord t) {
-        for (int i = t.mActivities.size() - 1; i >= 0; --i) {
-            final ActivityRecord r = t.mActivities.get(i);
+        for (int i = t.getChildCount() - 1; i >= 0; --i) {
+            final ActivityRecord r = t.getChildAt(i);
             if (r.visible && !r.finishing) {
                 return true;
             }