AppEngine.applicationId does not parse appid correctly

[matthewblain]

The method AppEngine.applicationId parses the App ID using a mechanism which is not correct in all scenarios.

It should either use, or copy the convention used in, the AppIdentityService.

A full appid is partition~domain:id. "s~" is a US based High Replication Datastore. Other common partitions include "e", "o", and "dev". There may be others, or even appid without a partition. Note that the lack of a partition is highly unlikely as the master-slave datastore has been deprecated for many years.

https://cloud.google.com/appengine/docs/standard/java/javadoc/com/google/appengine/api/appidentity/AppIdentityService

    AppIdentityService.ParsedAppId parsedId = getAppIdentityService().parseFullAppId(id);
    return parsedId.getId();

https://github.com/presentco/present-engine/blob/master/src/main/java/present/engine/AppEngine.java#L36