Update sample code in README

[mbernson]

Description

In modern Android, the flag PendingIntent.FLAG_MUTABLE is required. Otherwise the intent will not return any data when authentication is completed or canceled.

I've updated the code sample in the README to reflect this.

Motivation and Context

I'm a beginning Android developer and I was stuck on this for a while until I realised my error.

Checklist

• I read the Contribution Guidelines
• I signed the CLA and WG Agreements
• I ran, updated and added unit tests as necessary.
• I verified the contribution matches existing coding style.
• I updated the documentation if necessary.

[PhilippNowak96]

Since this flag was added in API 31, you should add some check, e.g.

val intentFlags = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
    PendingIntent.FLAG_MUTABLE
} else {
    0
}

See also Android Docs

[mbernson]

Fair enough! Adding all that info to the README would not be relevant to AppAuth and the user of the library should "RTFM". :)