kokoro: Increase xds-k8s timeout to 3 hours

The addition of the authz tests in 0d34572 is causing the tests to
exceed their timeout. By itself, the authz test takes about an hour in
this environment. Before the authz tests, xds-k8s was taking an hour
and a half.

buildscripts/kokoro/xds-k8s.cfg

@@ -2,7 +2,7 @@
 
 # Location of the continuous shell script in repository.
 build_file: "grpc-java/buildscripts/kokoro/xds-k8s.sh"
-timeout_mins: 120
+timeout_mins: 180
 
 action {
   define_artifacts {