Support for adding a Google Enterprise Account using new method introduced by Better Together Enterprise

Better Together Enterprise has introduced a new method for authenticating the end user and adding an account to the device depending upon the settings configured by the IT admin in the Google Admin Console. I'm adding support for this into TestDPC to allow EMMs to use it for manual testing, and to serve as a reference implementation for 3rd party DPC developers.

This feature makes use of the DPC support library, which is only available to members of the Android Enterprise EMM Community and so cannot be added to this repository. Instead I will add a fake that throws an exception if called, and will determine at runtime whether the real or fake library is present and disable the feature as necessary. Community members can replace the fake with the real library and re-build.