Patch proposal - Polishing this sample

[mariogrx]

I'd like to contribute by cleaning up this sample in the following way:

1. To update README.md as it refers to outdated instructions (Ids.java instead of Constants.java).
2. To merge the two directories with duplicate source code of the Endpoints (example/helloendpoints vs google/devrel/samples/helloendpoints).
3. To add the AuthLevel option of Endpoints to the Example.

[oalami]

Hi @mariogrx. Thanks! That sounds great, please make sure you've signed the CLA and send a PR.