Integrate Google Play Games SDK into Android Commerce Sample

[briantre]

Changes proposed in this Pull Request:
This updates the PlayFab Commerce sample for Android to use the v2 version of the Google Play Games SDK. Since the sample hadn't been updated in quite some time then this work also required moving the sample to the androidx framework.

This PR is currently in "Draft" because it is not yet functional with PlayFab. The following are known issues:

1. PlayFab does not currently support logging in using Google Play Games identities. It only supports Google Account identities. The previous version of this app used the "Google Sign In SDK" which enabled signing in only with Google Accounts. The version of the app in this PR only allows signing in with Google Play Games Player identities and there doesn't appear to be a way to map a Google Play Games Player to a Google Account.
2. I updated the server_client_id in res/values/strings.xml to point to a different server application. This is because I needed to create new web app credentials in the Google Cloud Platform project. This new server Id is not associated with the same PlayFab title. This means we'd need to create a new title in PlayFab and associate it with the new web application created in Google. This would mean that any existing users of the sample would lose any inventory they may have purchased.