Harden notification channels.

There's odd corner cases where channels can be duplicated. This commit
adds some hard checks where we trim any dead channels, and unset any
notification channels from recipients whose notification channel isn't
present in the system settings.

src/org/thoughtcrime/securesms/RecipientPreferenceActivity.java

@@ -282,6 +282,7 @@ public class RecipientPreferenceActivity extends PassphraseRequiredActionBarActi
               RecipientDatabase db = DatabaseFactory.getRecipientDatabase(getContext());
               db.setMessageRingtone(recipient, NotificationChannels.getMessageRingtone(context, recipient));
               db.setMessageVibrate(recipient, NotificationChannels.getMessageVibrate(context, recipient) ? VibrateState.ENABLED : VibrateState.DISABLED);
+              NotificationChannels.ensureCustomChannelConsistency(context);
               return null;
             }
           }.executeOnExecutor(AsyncTask.SERIAL_EXECUTOR);