update ActionCodeSettings snippets

Author: kevinthecheung

auth/app/src/main/java/com/google/firebase/quickstart/auth/MainActivity.java

@@ -340,6 +340,7 @@ public void buildActionCodeSettings() {
                         .setUrl("https://www.example.com/finishSignUp?cartId=1234")
                         // This must be true
                         .setHandleCodeInApp(true)
+                        .setLinkDomain("custom-domain.com")
                         .setIOSBundleId("com.example.ios")
                         .setAndroidPackageName(
                                 "com.example.android",

auth/app/src/main/java/com/google/firebase/quickstart/auth/kotlin/MainActivity.kt

@@ -280,6 +280,7 @@ abstract class MainActivity : AppCompatActivity() {
             url = "https://www.example.com/finishSignUp?cartId=1234"
             // This must be true
             handleCodeInApp = true
+            setLinkDomain("custom-domain.com")
             setIOSBundleId("com.example.ios")
             setAndroidPackageName(
                 "com.example.android",