Use the correct object to guard |mConversationsListeners|

The variable is marked as @GuardedBy("mLock"), but actually guarded
by itself.

Bug: 276714246
Test: m
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:cae843c3878eff70ae223c3e22b19558cb594b04)
Merged-In: I383e6c42f16f747dd80d604671b369fa491376cb
Change-Id: I383e6c42f16f747dd80d604671b369fa491376cb

services/people/java/com/android/server/people/data/DataManager.java

@@ -1339,7 +1339,7 @@ public class DataManager {
     /** Adds {@code listener} to be notified on conversation changes. */
     public void addConversationsListener(
             @NonNull PeopleService.ConversationsListener listener) {
-        synchronized (mConversationsListeners) {
+        synchronized (mLock) {
             mConversationsListeners.add(Objects.requireNonNull(listener));
         }
     }