alts: add Google Default Channel implementation (#4742)

alts: add Google Default Channel implementation

diff --git a/build.gradle b/build.gradle
index 7129629..21d0016 100644
--- a/build.gradle
+++ b/build.gradle
@@ -205,6 +205,7 @@
             oauth_client: 'com.google.auth:google-auth-library-oauth2-http:0.9.0',
             google_api_protos: 'com.google.api.grpc:proto-google-common-protos:1.0.0',
             google_auth_credentials: 'com.google.auth:google-auth-library-credentials:0.9.0',
+            google_auth_oauth2_http: 'com.google.auth:google-auth-library-oauth2-http:0.9.0',
             okhttp: 'com.squareup.okhttp:okhttp:2.5.0',
             okio: 'com.squareup.okio:okio:1.13.0',
             opencensus_api: "io.opencensus:opencensus-api:${opencensusVersion}",