Fix invalid `equals` in NotificationChannels.channelExists

`DEFAULT_CHANNEL_ID` is a String, but `channel` is a NotificationChannel. Equals will therefore always return `false`. I think my fix (using `getId()`) is what was intended.

src/org/thoughtcrime/securesms/notifications/NotificationChannels.java

@@ -517,7 +517,7 @@ public class NotificationChannels {
 
   @TargetApi(26)
   private static boolean channelExists(@Nullable NotificationChannel channel) {
-    return channel != null && !NotificationChannel.DEFAULT_CHANNEL_ID.equals(channel);
+    return channel != null && !NotificationChannel.DEFAULT_CHANNEL_ID.equals(channel.getId());
   }
 
   private interface ChannelUpdater {