Drop dependency on App Engine SDK

[yuri-sergiichuk]

There is no good reason to have the App Engine SDK as the implementation dependency in our libraries.

The SDK itself provides APIs that one can use only in the App Engine 8 environment and has an enormous 15mb size. Also, while in many projects we're going forward with Java 11 and App Engine 11, we'll no longer be able to use the App Engine SDK APIs.