chore(app): lint cleanups (const constructors, local function)

lib/logic/updater.dart

@@ -189,7 +189,7 @@ class Updater {
     }
 
     try {
-      final result = await MethodChannel('com.revoltvpn.app/installer')
+      final result = await const MethodChannel('com.revoltvpn.app/installer')
           .invokeMethod<String>('getInstallerPackage');
       _cachedSource = (result == 'com.android.vending')
           ? InstallSource.playStore