notif: Remove legacy PendingIntentFlag.updateCurrent flag

Docs: https://developer.android.com/reference/android/app/PendingIntent#FLAG_UPDATE_CURRENT

According to the documentation, this flag is only necessary when we want
to update the extras data without causing a new PendingIntent entry to
be created. Since we no longer pass any extras data, this flag is no
longer required.

Author: rajveermalviya

lib/notifications/display.dart

@@ -192,10 +192,7 @@ class NotificationDisplayManager {
         // The intent data URL is distinct for each conversation, so this value
         // doesn't matter.
         requestCode: 0,
-
-        // TODO is setting PendingIntentFlag.updateCurrent OK?
-        //   (That's a legacy of `flutter_local_notifications`.)
-        flags: PendingIntentFlag.immutable | PendingIntentFlag.updateCurrent,
+        flags: PendingIntentFlag.immutable,
         intent: AndroidIntent(
           action: IntentAction.view,
           dataUrl: intentDataUrl.toString(),

test/notifications/display_test.dart

@@ -225,8 +225,7 @@ void main() {
       final expectedGroupKey = '${data.realmUrl}|${data.userId}';
       final expectedId =
         NotificationDisplayManager.notificationIdAsHashOf(expectedTag);
-      const expectedIntentFlags =
-        PendingIntentFlag.immutable | PendingIntentFlag.updateCurrent;
+      const expectedIntentFlags = PendingIntentFlag.immutable;
       final expectedSelfUserKey = '${data.realmUrl}|${data.userId}';
       final expectedIntentDataUrl = NotificationOpenPayload(
         realmUrl: data.realmUrl,