android, notif: Fix custom sound is not played API >= 26.

From API >= 26, Android added support of Notification channels. So now
notifications sounds are handelled by channels, not by notification
itself. So set sound to our one and only defualt channel.

Fixes: zulip#3150

android/app/src/main/java/com/zulipmobile/notifications/GCMPushNotifications.java

@@ -50,6 +50,9 @@ public static void createNotificationChannel(Context context) {
             CharSequence name = context.getString(R.string.notification_channel_name);
             int importance = NotificationManager.IMPORTANCE_HIGH;
             NotificationChannel channel = new NotificationChannel(CHANNEL_ID, name, importance);
+            AudioAttributes audioAttr = new AudioAttributes.Builder()
+                    .setUsage(AudioAttributes.USAGE_NOTIFICATION).build();
+            channel.setSound(Uri.parse(ContentResolver.SCHEME_ANDROID_RESOURCE +"://" + context.getPackageName() + NOTIFICATION_SOUND_PATH), audioAttr);
             getNotificationManager(context).createNotificationChannel(channel);
         }
     }
@@ -173,6 +176,7 @@ private static Notification.Builder getNotificationBuilder(
             builder.addAction(action);
         }
 
+        // Note: This code only set custom sound for API < 26 (from higher API sound is managed by channels)
         if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.LOLLIPOP) {
             AudioAttributes audioAttr = new AudioAttributes.Builder()
                     .setUsage(AudioAttributes.USAGE_NOTIFICATION).build();