java_grpc_library.bzl: Pre-migrate for Bazel incompatible_use_toolchain_providers_in_java_common

This doesn't actually yet work with
--incompatible_use_toolchain_providers_in_java_common, as Bazel 0.23 didn't
include enough pieces. But this will work in 0.24 with the flag flipped. In
both cases it will continue working if the flag is not specified.

See https://github.com/grpc/grpc-java/issues/5383#issuecomment-470357965 and
https://github.com/bazelbuild/bazel/issues/7186

java_grpc_library.bzl

@@ -1,3 +1,5 @@
+load("@bazel_tools//tools/jdk:toolchain_utils.bzl", "find_java_runtime_toolchain", "find_java_toolchain")
+
 # "repository" here is for Bazel builds that span multiple WORKSPACES.
 def _path_ignoring_repository(f):
     if len(f.owner.workspace_root) == 0:
@@ -40,8 +42,8 @@ def _java_rpc_library_impl(ctx):
 
     java_info = java_common.compile(
         ctx,
-        java_toolchain = ctx.attr._java_toolchain,
-        host_javabase = ctx.attr._host_javabase,
+        java_toolchain = find_java_toolchain(ctx, ctx.attr._java_toolchain),
+        host_javabase = find_java_runtime_toolchain(ctx, ctx.attr._host_javabase),
         source_jars = [srcjar],
         output_source_jar = ctx.outputs.srcjar,
         output = ctx.outputs.jar,