Car: Avoid Errorprone SelfAssignment error

A checked self-assignment is still treated as a potentially bug-prone
self-assignment. Avoid.

Bug: 72076216
Test: m javac-check RUN_ERROR_PRONE=true
Change-Id: I6a3ef4f898a7242b48e67e2ecae06015407678e9
diff --git a/service/src/com/android/car/CarDiagnosticService.java b/service/src/com/android/car/CarDiagnosticService.java
index 809439c..ff0b7a9 100644
--- a/service/src/com/android/car/CarDiagnosticService.java
+++ b/service/src/com/android/car/CarDiagnosticService.java
@@ -600,7 +600,7 @@
 
         @Override
         CarDiagnosticEvent update(@NonNull CarDiagnosticEvent newEvent) {
-            newEvent = Objects.requireNonNull(newEvent);
+            Objects.requireNonNull(newEvent);
             if((null == mLastEvent) || mLastEvent.isEarlierThan(newEvent))
                 mLastEvent = newEvent;
             return mLastEvent;