Refactor App to work with AppNavigator

[EstebanOlmedo]

Refactor App.tsx to work with the new AppNavigator component. Add App.Navigator component which will be used to change between screens in the app. Move and rename HomeScreenComponent and SignUpScreenComponent to HomeScreen and SignUpScreen under app/screens directory, this will help to differentiate between screens and components. Change the way to export components, instead of use

    export default function <component>

use

    const <component> () => {}
    export default <component>;

Add minimal navigation.