Move the call to flush() to avoid a race condition

Bug: 221393932
Test: atest PersistentDataStoreTest
Test: Manually change brightness, run 'adb shell dumpsys display | grep -A30 PersistentDataStore' and check that the brightness value has updated. Also reboot the device and check that the brightness value stays the same.
Change-Id: I6f0b958a7db832a0ebe5e9f2ebc434ac92f47a8a

services/core/java/com/android/server/display/PersistentDataStore.java

@@ -463,6 +463,7 @@ final class PersistentDataStore {
 
             TypedXmlSerializer serializer = Xml.resolveSerializer(os);
             saveToXml(serializer);
+            serializer.flush();
 
             mHandler.removeCallbacksAndMessages(/* token */ null);
             mHandler.post(() -> {
@@ -481,8 +482,6 @@ final class PersistentDataStore {
                     }
                 }
             });
-
-            serializer.flush();
         } catch (IOException ex) {
             Slog.w(TAG, "Failed to process the XML serializer.", ex);
         }