Declare gax depedency as api.
Declare gax depedency as api.

This exposes the gax-grpc library as an api dependency and fixes maven
builds. Fixes https://issuetracker.google.com/129261445

Change-Id: I7b9c150980b7d5d47b5e6718a7f9ada363335fb5

google/java-photoslibrary