DarkMode: move to shared preferences listener

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

app/src/main/java/com/wireguard/android/Application.java

@@ -23,7 +23,6 @@ import com.wireguard.android.model.TunnelManager;
 import com.wireguard.android.util.AsyncWorker;
 import com.wireguard.android.util.RootShell;
 import com.wireguard.android.util.ToolsInstaller;
-import com.wireguard.android.util.Topic;
 
 import java.io.File;
 import java.util.concurrent.Executor;
@@ -68,8 +67,6 @@ public class Application extends android.app.Application {
         ToolsInstaller getToolsInstaller();
 
         TunnelManager getTunnelManager();
-
-        Topic getThemeChangeTopic();
     }
 
     @Qualifier
@@ -119,13 +116,6 @@ public class Application extends android.app.Application {
             return new FileConfigStore(context);
         }
 
-        @ApplicationScope
-        @Provides
-        public static Topic getThemeChangeTopic() {
-            return new Topic();
-        }
-
-
         @ApplicationScope
         @Provides
         public static Executor getExecutor() {