chore: use java helper for templates (#114)

Author: chingor13

synth.py

@@ -64,9 +64,7 @@
 java.format_code('grpc-google-cloud-spanner-admin-instance-v1/src')
 java.format_code('proto-google-cloud-spanner-admin-instance-v1/src')
 
-common_templates = gcp.CommonTemplates()
-templates = common_templates.java_library()
-s.copy(templates, excludes=[
+java.common_templates(excludes=[
     'README.md',
     '.kokoro/continuous/common.cfg',
     '.kokoro/nightly/common.cfg',