Flex environment deployments still have runtime dependency on AppEngine SDK

[clementdenis]

f21ca93 was not enough to remove runtime dependecies on the App Engine SDK.

Steps to reproduce:

• Deploy an API using Cloud Endpoints v2 on a flex environement, making sure the App Engine SDK jar is not deployed
• Call the API using an unauthorized client id on a method requiring authentication (User injected)
• Get the exception below:

java.lang.NoClassDefFoundError: com/google/appengine/api/oauth/OAuthRequestException
at com.google.api.server.spi.auth.EndpointsAuthenticator.<init>(EndpointsAuthenticator.java:40)
at com.google.api.server.spi.request.Auth.<clinit>(Auth.java:48)
at com.google.api.server.spi.request.ServletRequestParamReader.getUser(ServletRequestParamReader.java:192)
at com.google.api.server.spi.request.ServletRequestParamReader.deserializeParams(ServletRequestParamReader.java:137)
at com.google.api.server.spi.request.RestServletRequestParamReader.read(RestServletRequestParamReader.java:124)
at com.google.api.server.spi.SystemService.invokeServiceMethod(SystemService.java:349)
at com.google.api.server.spi.handlers.EndpointsMethodHandler$RestHandler.handle(EndpointsMethodHandler.java:119)
at com.google.api.server.spi.handlers.EndpointsMethodHandler$RestHandler.handle(EndpointsMethodHandler.java:102)
at com.google.api.server.spi.dispatcher.PathDispatcher.dispatch(PathDispatcher.java:49)
at com.google.api.server.spi.EndpointsServlet.service(EndpointsServlet.java:71)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)

The dependency is in com.google.api.server.spi.auth.GoogleAppEngineAuthenticator.

A possible fix here: AODocs@dca92b2
Next step is probably to set the App Engine SDk dependency as "compileOnly"