[Enhancement] Improve API Key Setup and README

[sorianog]

In MainActivity.java, add the following:

private static final String API_KEY = "your-api-key-here";

private void initializeDeepAR() {
    deepAR = new DeepAR(this);
    deepAR.setLicenseKey(API_KEY);
    deepAR.initialize(this, this);

}

For README:

• Add screenshots of API Key Generation/How-To