add some low level files for android

Author: AhmedAbdoElhawary

android/.gitignore

@@ -5,9 +5,10 @@ gradle-wrapper.jar
 /gradlew.bat
 /local.properties
 GeneratedPluginRegistrant.java
+.cxx/
 
 # Remember to never publicly share your keystore.
-# See https://flutter.dev/docs/deployment/android#reference-the-keystore-from-the-app
+# See https://flutter.dev/to/reference-keystore
 key.properties
 **/*.keystore
 **/*.jks

android/app/src/main/AndroidManifest.xml

@@ -2,6 +2,8 @@
     <application
         android:label="algorithm_visualizer"
         android:name="${applicationName}"
+        android:enableOnBackInvokedCallback="true"
+
         android:icon="@mipmap/ic_launcher">
         <activity
             android:name=".MainActivity"