notif: Enable autoCancel to dismiss notifications when tapped

Set autoCancel flag for notifications, ensuring they automatically
disappear from the panel when tapped.

See:
  https://developer.android.com/reference/androidx/core/app/NotificationCompat.Builder#setAutoCancel(boolean)

Also, zulip-mobile does this already:
  https://github.com/zulip/zulip-mobile/blob/e352f563ecf2fa9b09b688d5a65b6bc89b0358bc/android/app/src/main/java/com/zulipmobile/notifications/NotificationUiManager.kt#L353

Author: rajveermalviya

lib/notifications/display.dart

@@ -142,6 +142,7 @@ class NotificationDisplayManager {
         // TODO this doesn't set the Intent flags we set in zulip-mobile; is that OK?
         //   (This is a legacy of `flutter_local_notifications`.)
         ),
+      autoCancel: true,
     );
 
     await ZulipBinding.instance.androidNotificationHost.notify(

test/notifications/display_test.dart

@@ -131,7 +131,7 @@ void main() {
             ..groupKey.equals(expectedGroupKey)
             ..isGroupSummary.isNull()
             ..inboxStyle.isNull()
-            ..autoCancel.isNull()
+            ..autoCancel.equals(true)
             ..contentIntent.which((it) => it.isNotNull()
               ..requestCode.equals(expectedId)
               ..flags.equals(expectedIntentFlags)