Link priority setting to message channel setting on Oreo+

When using notification channels, us setting priority actually has
no effect. So instead of having a non-functional setting, we've
routed the notification priority setting to go to the system
notification channel settings page for our Messages channel.

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

@@ -30,8 +30,11 @@ public class MultipleRecipientNotificationBuilder extends AbstractNotificationBu
     setContentTitle(context.getString(R.string.app_name));
     setContentIntent(PendingIntent.getActivity(context, 0, new Intent(context, ConversationListActivity.class), 0));
     setCategory(NotificationCompat.CATEGORY_MESSAGE);
-    setPriority(TextSecurePreferences.getNotificationPriority(context));
     setGroupSummary(true);
+
+    if (!NotificationChannels.supported()) {
+      setPriority(TextSecurePreferences.getNotificationPriority(context));
+    }
   }
 
   public void setMessageCount(int messageCount, int threadCount) {

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

@@ -26,7 +26,7 @@ public class NotificationChannels {
    * ignored for API < 26.
    */
   public static void create(@NonNull Context context) {
-    if (Build.VERSION.SDK_INT < 26) {
+    if (!supported()) {
       return;
     }
 
@@ -47,4 +47,8 @@ public class NotificationChannels {
       notificationManager.deleteNotificationChannel(APP_UPDATES);
     }
   }
+
+  public static boolean supported() {
+    return Build.VERSION.SDK_INT >= 26;
+  }
 }

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

@@ -21,7 +21,6 @@ public class PendingMessageNotificationBuilder extends AbstractNotificationBuild
 
     setSmallIcon(R.drawable.icon_notification);
     setColor(context.getResources().getColor(R.color.textsecure_primary));
-    setPriority(TextSecurePreferences.getNotificationPriority(context));
     setCategory(NotificationCompat.CATEGORY_MESSAGE);
 
     setContentTitle(context.getString(R.string.MessageNotifier_pending_signal_messages));
@@ -31,5 +30,9 @@ public class PendingMessageNotificationBuilder extends AbstractNotificationBuild
     setContentIntent(PendingIntent.getActivity(context, 0, intent, 0));
     setAutoCancel(true);
     setAlarms(null, RecipientDatabase.VibrateState.DEFAULT);
+
+    if (!NotificationChannels.supported()) {
+      setPriority(TextSecurePreferences.getNotificationPriority(context));
+    }
   }
 }

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

@@ -52,8 +52,11 @@ public class SingleRecipientNotificationBuilder extends AbstractNotificationBuil
 
     setSmallIcon(R.drawable.icon_notification);
     setColor(context.getResources().getColor(R.color.textsecure_primary));
-    setPriority(TextSecurePreferences.getNotificationPriority(context));
     setCategory(NotificationCompat.CATEGORY_MESSAGE);
+
+    if (!NotificationChannels.supported()) {
+      setPriority(TextSecurePreferences.getNotificationPriority(context));
+    }
   }
 
   public void setThread(@NonNull Recipient recipient) {