fix(firebase_auth): Move communication to EventChannels (no-nullsafety)

packages/firebase_auth/firebase_auth/android/src/main/java/io/flutter/plugins/firebase/auth/AuthStateChannelStreamHandler.java

@@ -0,0 +1,58 @@
+package io.flutter.plugins.firebase.auth;
+
+import static io.flutter.plugins.firebase.auth.FlutterFirebaseAuthPlugin.parseFirebaseUser;
+
+import com.google.firebase.auth.FirebaseAuth;
+import com.google.firebase.auth.FirebaseAuth.AuthStateListener;
+import com.google.firebase.auth.FirebaseUser;
+import io.flutter.plugin.common.EventChannel.EventSink;
+import io.flutter.plugin.common.EventChannel.StreamHandler;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+public class AuthStateChannelStreamHandler implements StreamHandler {
+
+  private final FirebaseAuth firebaseAuth;
+  private AuthStateListener authStateListener;
+
+  public AuthStateChannelStreamHandler(FirebaseAuth firebaseAuth) {
+    this.firebaseAuth = firebaseAuth;
+  }
+
+  @Override
+  public void onListen(Object arguments, EventSink events) {
+    Map<String, Object> event = new HashMap<>();
+    event.put(Constants.APP_NAME, firebaseAuth.getApp().getName());
+
+    final AtomicBoolean initialAuthState = new AtomicBoolean(true);
+
+    authStateListener =
+        auth -> {
+          if (initialAuthState.get()) {
+            initialAuthState.set(false);
+            return;
+          }
+
+          FirebaseUser user = auth.getCurrentUser();
+
+          if (user == null) {
+            event.put(Constants.USER, null);
+          } else {
+            event.put(Constants.USER, parseFirebaseUser(user));
+          }
+
+          events.success(event);
+        };
+
+    firebaseAuth.addAuthStateListener(authStateListener);
+  }
+
+  @Override
+  public void onCancel(Object arguments) {
+    if (authStateListener != null) {
+      firebaseAuth.removeAuthStateListener(authStateListener);
+      authStateListener = null;
+    }
+  }
+}

packages/firebase_auth/firebase_auth/android/src/main/java/io/flutter/plugins/firebase/auth/Constants.java

@@ -56,7 +56,6 @@ public class Constants {
   public static final String ACCESS_TOKEN = "accessToken";
   public static final String CODE = "code";
   public static final String RAW_NONCE = "rawNonce";
-  public static final String HANDLE = "handle";
   public static final String EMAIL_LINK = "emailLink";
   public static final String VERIFICATION_ID = "verificationId";
   public static final String SMS_CODE = "smsCode";