docs: Update extended activity related code

demo/AngularApp/app/activity.android.ts

@@ -2,9 +2,14 @@ import {setActivityCallbacks, AndroidActivityCallbacks} from "ui/frame";
 
 @JavaProxy("org.myApp.MainActivity")
 class Activity extends android.support.v7.app.AppCompatActivity {
+    public isNativeScriptActivity: boolean;
     private _callbacks: AndroidActivityCallbacks;
 
     protected onCreate(savedInstanceState: any): void { // android.os.Bundle
+        // Set isNativeScriptActivity in onCreate (as done in the original NativeScript activity code).
+        // The JS constructor might not be called because the activity is created from Android.
+        this.isNativeScriptActivity = true;
+
         if (!this._callbacks) {
             setActivityCallbacks(this);
         }

demo/JavaScriptApp/app/activity.android.js

@@ -3,6 +3,10 @@ const frame = require("ui/frame");
 const superProto = android.support.v7.app.AppCompatActivity.prototype;
 android.support.v7.app.AppCompatActivity.extend("org.myApp.MainActivity", {
     onCreate: function(savedInstanceState) {
+        // Set isNativeScriptActivity in onCreate.
+        // The JS constructor might not be called because the activity is created from Android.
+        this.isNativeScriptActivity = true;
+
         if(!this._callbacks) {
             frame.setActivityCallbacks(this);
         }

demo/TypeScriptApp/app/activity.android.ts

@@ -2,9 +2,14 @@ import {setActivityCallbacks, AndroidActivityCallbacks} from "ui/frame";
 
 @JavaProxy("org.myApp.MainActivity")
 class Activity extends android.support.v7.app.AppCompatActivity {
+    public isNativeScriptActivity: boolean;
     private _callbacks: AndroidActivityCallbacks;
-
+    
     protected onCreate(savedInstanceState: any): void { // android.os.Bundle
+        // Set isNativeScriptActivity in onCreate (as done in the original NativeScript activity code).
+        // The JS constructor might not be called because the activity is created from Android.
+        this.isNativeScriptActivity = true;
+
         if (!this._callbacks) {
             setActivityCallbacks(this);
         }