Bump grpc-java to 1.18.0

Requires bumping other dependencies in deps.json such as Netty (4.1.32)
and error_prone_annotations. Includes also fixes detected by updated
errorprone.

Change-Id: Ic2bd86931cc89bfb2cf1a4cb11a4586bc8cac608
diff --git a/tools/build/bazel/variables.bzl b/tools/build/bazel/variables.bzl
index 22ba133..8543189 100644
--- a/tools/build/bazel/variables.bzl
+++ b/tools/build/bazel/variables.bzl
@@ -5,5 +5,5 @@
 APP_PREFIX = ONOS_GROUP_ID + "."
 DEFAULT_APP_CATEGORY = "Utility"
 
-GRPC_JAVA_VERSION = "1.14.0"
+GRPC_JAVA_VERSION = "1.18.0"
 PROTOBUF_VERSION = "3.6.1.3"