Enable error-prone checks by default

Based on recent work by Dave Borowitz on the Gerrit project [1, 2].

The warnings/error configuration is unchanged, but now the checks are
enabled by default during the build rather than having to be manually
invoked.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/225653
[2] https://gerrit-review.googlesource.com/c/gerrit/+/225714

Change-Id: I112146911d3067bb5283cdf7b188be73f8f8f1e8

Author: dpursehouse

.bazelrc

@@ -3,6 +3,7 @@ build --repository_cache=~/.gerritcodereview/bazel-cache/repository
 build --experimental_strict_action_env
 build --action_env=PATH
 build --disk_cache=~/.gerritcodereview/bazel-cache/cas
+build --java_toolchain //tools:error_prone_warnings_toolchain
 
 test --build_tests_only
 test --test_output=errors

tools/BUILD

@@ -14,8 +14,10 @@ default_java_toolchain(
     visibility = ["//visibility:public"],
 )
 
-# This EP warnings list borrowed from here:
+# Error Prone errors enabled by default; see ../.bazelrc for how this is
+# enabled. This warnings list is originally based on:
 # https://github.com/bazelbuild/BUILD_file_generator/blob/master/tools/bazel_defs/java.bzl
+# However, feel free to add any additional errors. Thus far they have all been pretty useful.
 java_package_configuration(
     name = "error_prone",
     javacopts = [