Suggested updates for XE16 and AppEngine udpates

[johnfontaine]

There are a few issues in this sample code that I have cleaned up.

1. The AuthUtil was CredentialStore which is deprecated. I've replaced it with StoredCredential and the AppEngineDataStore. This enables deprecation of ListableMemoryCredentialStore.java, though it should be retained for migration purposes.
2. The AuthServlet was calling payload.getUserId() which is no longer available and has been replaced with getSubject()
3. I've updated the api calls in the pom.xml to use the latest version of Google API and the Latest mirror api.
4. I've added AuthUtil.installMirrorAccount. This provides an example for GDK authentication via Mirror for apps.