Reapply "AudioFlinger: Clean up NotificationClients"

This reverts commit bcd19ea395cb1112454870b97d16020d8aeb6bbc.

Test: atest AudioTrackTest
Flag: EXEMPT refactor
Bug: 350114693
Change-Id: I1c1e579066b04b71f8dd44d42ab8a96fcb8690a0
diff --git a/services/audioflinger/AudioFlinger.h b/services/audioflinger/AudioFlinger.h
index de01756..7c58c96 100644
--- a/services/audioflinger/AudioFlinger.h
+++ b/services/audioflinger/AudioFlinger.h
@@ -698,8 +698,7 @@
 
     DefaultKeyedVector<audio_io_handle_t, sp<IAfRecordThread>> mRecordThreads GUARDED_BY(mutex());
 
-    DefaultKeyedVector<pid_t, sp<NotificationClient>> mNotificationClients
-            GUARDED_BY(clientMutex());
+    std::map<pid_t, sp<NotificationClient>> mNotificationClients GUARDED_BY(clientMutex());
 
                 // updated by atomic_fetch_add_explicit
     volatile atomic_uint_fast32_t mNextUniqueIds[AUDIO_UNIQUE_ID_USE_MAX];  // ctor init