Remove Unable to get TurboModule for DialogManagerAndroid warning for iOS

Summary:
The import of DialogManagerAndroid in [Alert.js](https://fburl.com/diffusion/n57e4l50) causes "Unable to get iOS TurboModule for DialogManagerAndroid" log.

Don't call ` TurboModuleRegistry.get` on DialogManagerAndroid when the Platform is iOS.

Changelog: [Internal]

Reviewed By: fkgozali

Differential Revision: D27376204

fbshipit-source-id: 948da5c162100ffe196d5e0a54dc8b85659239ae

Author: p-sun

Libraries/Alert/Alert.js

@@ -9,9 +9,7 @@
  */
 
 import Platform from '../Utilities/Platform';
-import NativeDialogManagerAndroid, {
-  type DialogOptions,
-} from '../NativeModules/specs/NativeDialogManagerAndroid';
+import type {DialogOptions} from '../NativeModules/specs/NativeDialogManagerAndroid';
 import RCTAlertManager from './RCTAlertManager';
 
 export type AlertType =
@@ -48,6 +46,8 @@ class Alert {
     if (Platform.OS === 'ios') {
       Alert.prompt(title, message, buttons, 'default');
     } else if (Platform.OS === 'android') {
+      const NativeDialogManagerAndroid = require('../NativeModules/specs/NativeDialogManagerAndroid')
+        .default;
       if (!NativeDialogManagerAndroid) {
         return;
       }

Libraries/TurboModule/TurboModuleRegistry.js

@@ -28,7 +28,7 @@ function requireModule<T: TurboModule>(name: string): ?T {
     const module: ?T = turboModuleProxy(name);
     if (module == null) {
       // Common fixes: Verify the TurboModule is registered in the native binary, and adopts the code generated type-safe Spec base class.
-      // May be caused by importing legacy modules in JS.
+      // Safe to ignore when caused by importing legacy modules that are unused.
       console.info(
         'Unable to get TurboModule for ' +
           name +