Revert "Revert "[AC] Fix NPE. If the client sends multiple unregister requests at the same time, the client request could be null before the next request is processed.""

This reverts commit f470e292d8.

Reason for revert: Fixed warning from presubmit

Change-Id: I3e1ea17a2e149b548885666d9e8418647b96439e
(cherry picked from commit b93d2ce0d3)
Bug: 278891267
Test: make

services/core/java/com/android/server/ambientcontext/AmbientContextManagerService.java

@@ -60,6 +60,7 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Objects;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 
 /**
  * System service for managing {@link AmbientContextEvent}s.
@@ -595,7 +596,7 @@ public class AmbientContextManagerService extends
 
             synchronized (mLock) {
                 for (ClientRequest cr : mExistingClientRequests) {
-                    if (cr.getPackageName().equals(callingPackage)) {
+                    if ((cr != null) && cr.getPackageName().equals(callingPackage)) {
                         AmbientContextManagerPerUserService service =
                                 getAmbientContextManagerPerUserServiceForEventTypes(
                                         UserHandle.getCallingUserId(),