build.gradle: Add Google Client ID for Google auth.
build.gradle: Add Google Client ID for Google auth.

GOOGLE_CLIENT_ID is a public key, and has been published in the Zulip
server repository since Zulip was open sourced, so I think it makes
sense to add it here to simplify the release process.

prati0100/zulip-android,saketkumar95/zulip-android,vishwesh3/zulip-android,prati0100/zulip-android,abhaymaniyar/zulip-android,Sam1301/zulip-android,zulip/zulip-android,abhaymaniyar/zulip-android,saketkumar95/zulip-android,Sam1301/zulip-android