Fix a bug with the demoProjectId arg to Firebase.initializeApp()

If platform specific configuration files exist for the project (i.e.
GoogleService-Info.plist for iOS), the default `MethodChannelFirebaseApp` is
initialized using the configuration it specifies. This is fine since multiple
`FirebaseApp` instances can exist, however when
`MethodChannelFirebase.initializeApp()` is called it uses the default app name
(`[DEFAULT]`, from the code) if no name is passed as an argument. Since the
default app is initialized using the platform specific configuration before
`initializeApp()` is called, the user provided options are ignored.

The solution is to ensure a non-null app name is passed to `initializeApp()` to
avoid conflicts with platform specific configuration files when the user intends
to use a "demo-" project for testing.

Since we should be providing a distinct app name for the demo app, the
allowance for mismatched options between the specified options and the existing
options for demo apps is also removed.

Author: emesterhazy

packages/firebase_core/firebase_core/lib/src/firebase.dart

@@ -53,6 +53,7 @@ class Firebase {
         platformString = 'web';
       }
       FirebaseAppPlatform app = await _delegate.initializeApp(
+        name: name ?? demoProjectId,
         options: FirebaseOptions(
           apiKey: '',
           appId: '1:1:$platformString:1',

packages/firebase_core/firebase_core_platform_interface/lib/src/method_channel/method_channel_firebase.dart

@@ -121,9 +121,7 @@ class MethodChannelFirebase extends FirebasePlatform {
       // check to see if options are roughly identical (so we don't unnecessarily
       // throw on minor differences such as platform specific keys missing
       // e.g. hot reloads/restarts).
-      if (defaultApp != null &&
-          _options != null &&
-          !_options.projectId.startsWith('demo-')) {
+      if (defaultApp != null && _options != null &&) {
         if (_options.apiKey != defaultApp.options.apiKey ||
             (_options.databaseURL != null &&
                 _options.databaseURL != defaultApp.options.databaseURL) ||