Add license checker config and kokoro presubmit

`license-checker` is a tool that verifies each file has contains a permitted license header.

See https://github.com/ben-clayton/license-checker for more information.

Also add missing licenses to presubmit scripts.

kokoro/macos/presubmit.sh

@@ -1,5 +1,19 @@
 #!/bin/bash
 
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
 set -e # Fail on any error.
 set -x # Display commands being run.
 
@@ -20,4 +34,4 @@ if [ "$BUILD_SYSTEM" == "cmake" ]; then
 else
     echo "Unknown build system: $BUILD_SYSTEM"
     exit 1
-fi
+fi