Upgraded sample app

[terryyoon-pdftron]

NSDK-168

Overview:

This PR replaces the sample app in the example folder with a more comprehensive app instead of a standalone PDF viewer in order to better demonstrate how the wrapper can be embedded in an existing app and how its API can be used.

The app is a simple file manager within the app's folder with the ability to add files from the device's local storage or from the web, plus other miscellaneous functionalities. It uses this package for opening PDF files and for converting Office files to PDF.

The app is built using Expo SDK 46 with bare workflow (ejected) and React Native version 0.69.4.

Relevant changes are in the file App.tsx and the files under the components directory (other changes are due to React Native version changes and using Expo). Testing future changes can now be done by modifying the PDFViewer.tsx file instead of App.js.

Testing:

• Checkout this branch in the outer repo.
• Delete node_modules, and run yarn install
• Within node_modules, delete react-native-pdftron, clone this repo and re-name it to react-native-pdftron.
• Run the app:
  • iOS: yarn ios
  • Android: yarn android

[sgong-pdftron]

@darrenchann could you please perform first round of review, thanks.