Don't create unnecessary objects.

Only create a new LinkedList if one doesn't exist already for the user.

Bug: n/a
Test: atest UsageStatsTest
diff --git a/services/usage/java/com/android/server/usage/UsageStatsService.java b/services/usage/java/com/android/server/usage/UsageStatsService.java
index 2fd6c42..78556ef 100644
--- a/services/usage/java/com/android/server/usage/UsageStatsService.java
+++ b/services/usage/java/com/android/server/usage/UsageStatsService.java
@@ -763,11 +763,12 @@
                 return;
             }
 
-            final LinkedList<Event> events = mReportedEvents.get(userId, new LinkedList<>());
-            events.add(event);
-            if (mReportedEvents.get(userId) == null) {
+            LinkedList<Event> events = mReportedEvents.get(userId);
+            if (events == null) {
+                events = new LinkedList<>();
                 mReportedEvents.put(userId, events);
             }
+            events.add(event);
             if (events.size() == 1) {
                 // Every time a file is persisted to disk, mReportedEvents is cleared for this user
                 // so trigger a flush to disk every time the first event has been added.