[gnome-break-timer] Remove Vala 0.49-specific slicing syntax

[Dylan McCall <dylanmccall src gnome org>]

commit bfc267d694f637df9aae9ce3289a3ef95ab9c17e
Author: Dylan McCall <dylan dylanmccall ca>
Date:   Mon Nov 30 18:12:45 2020 -0800

    Remove Vala 0.49-specific slicing syntax
    
    Vala 0.49.1 added support for empty start/end index in slice
    expressions, but we should support earlier versions of Vala for now.

 src/settings/BreakManager.vala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/src/settings/BreakManager.vala b/src/settings/BreakManager.vala
index 1b89eb7..cf08861 100644
--- a/src/settings/BreakManager.vala
+++ b/src/settings/BreakManager.vala
@@ -122,7 +122,8 @@ public class BreakManager : GLib.Object {
             return false;
         }
 
-        string sender_name = this.dbus_connection.unique_name.replace (".", "_")[1:];
+        string sender_name = this.dbus_connection.unique_name.replace (".", "_");
+        sender_name = sender_name[1:sender_name.length];
         string handle_token = "org_gnome_breaktimer%d".printf (
             GLib.Random.int_range (0, int.MAX)
         );