Make Linking.getInitialURL works properly even if current activity is not initialized (#24748)

Summary:
Solves #15961 - [ANDROID] Linking.getInitialURL() returns null after exiting the app with back button and using deep linking again (only in production!)

As the issue says, it seems that `currentActivity` is null in [IntentModule#getInitialURL](https://github.com/facebook/react-native/blob/1e8f3b11027fe0a7514b4fc97d0798d3c64bc895/ReactAndroid/src/main/java/com/facebook/react/modules/intent/IntentModule.java#L50). So I used the `InteractionManager` to wait until current activity to finish initializing.

[Android] [Fixed] - In `Linking.getInitialURL` method, use the `InteractionManager` to wait for the current activity to finish initializing.
Pull Request resolved: #24748

Differential Revision: D15258046

Pulled By: cpojer

fbshipit-source-id: 0ec1c873766ae90955d47e5e023556eb6c5d5fbb

Author: Injung Chung

Libraries/Linking/Linking.js

@@ -13,6 +13,7 @@
 const NativeEventEmitter = require('NativeEventEmitter');
 const NativeModules = require('NativeModules');
 const Platform = require('Platform');
+const InteractionManager = require('InteractionManager');
 
 const invariant = require('invariant');
 
@@ -87,7 +88,11 @@ class Linking extends NativeEventEmitter {
    * See https://facebook.github.io/react-native/docs/linking.html#getinitialurl
    */
   getInitialURL(): Promise<?string> {
-    return LinkingManager.getInitialURL();
+    return Platform.OS === 'android'
+      ? InteractionManager.runAfterInteractions().then(() =>
+          LinkingManager.getInitialURL(),
+        )
+      : LinkingManager.getInitialURL();
   }
 
   /*