bazel: Migrate java_grpc_library to use DefaultInfo (#12148)

We here address the following obstacles in grpc-java to using Bazel's
--incompatible_disable_target_default_provider_fields flag:

```
ERROR: /private/var/tmp/_bazel_dws/7fd3cd5077fbf76d9e2ae421c39ef7ed/external/googleapis+/google/devtools/build/v1/BUILD.bazel:81:18: in _java_grpc_library rule @@googleapis+//google/devtools/build/v1:build_java_grpc:
Traceback (most recent call last):
        File "/private/var/tmp/_bazel_dws/7fd3cd5077fbf76d9e2ae421c39ef7ed/external/grpc-java+/java_grpc_library.bzl", line 94, column 30, in _java_rpc_library_impl
                args.add(toolchain.plugin.files_to_run.executable, format = "--plugin=protoc-gen-rpc-plugin=%s")
Error: Accessing the default provider in this manner is deprecated and will be removed soon. It may be temporarily re-enabled by setting --incompatible_disable_target_default_provider_fields=false. See bazelbuild/bazel#20183 for details.
ERROR: /private/var/tmp/_bazel_dws/7fd3cd5077fbf76d9e2ae421c39ef7ed/external/googleapis+/google/devtools/build/v1/BUILD.bazel:81:18: Analysis of target '@@googleapis+//google/devtools/build/v1:build_java_grpc' failed
ERROR: Analysis of target '//src:bazel' failed; build aborted: Analysis failed
```

Author: dws

java_grpc_library.bzl

@@ -91,15 +91,15 @@ def _java_rpc_library_impl(ctx):
     srcjar = ctx.actions.declare_file("%s-proto-gensrc.jar" % ctx.label.name)
 
     args = ctx.actions.args()
-    args.add(toolchain.plugin.files_to_run.executable, format = "--plugin=protoc-gen-rpc-plugin=%s")
+    args.add(toolchain.plugin[DefaultInfo].files_to_run.executable, format = "--plugin=protoc-gen-rpc-plugin=%s")
     args.add("--rpc-plugin_out={0}:{1}".format(toolchain.plugin_arg, srcjar.path))
     args.add_joined("--descriptor_set_in", descriptor_set_in, join_with = ctx.configuration.host_path_separator)
     args.add_all(srcs, map_each = _path_ignoring_repository)
 
     ctx.actions.run(
-        inputs = depset(srcs, transitive = [descriptor_set_in, toolchain.plugin.files]),
+        inputs = depset(srcs, transitive = [descriptor_set_in, toolchain.plugin[DefaultInfo].files]),
         outputs = [srcjar],
-        executable = toolchain.protoc.files_to_run,
+        executable = toolchain.protoc[DefaultInfo].files_to_run,
         arguments = [args],
         use_default_shell_env = True,
         toolchain = None,