chore: use java helper for templates (#195)

We are cleaning up usage of common templates in synthtool. This allows us the cleanup usage in a centralized place rather than in 60+ repos

synth.py

@@ -14,12 +14,9 @@
 
 """This script is used to synthesize generated parts of this library."""
 
-import synthtool as s
-import synthtool.gcp as gcp
+import synthtool.languages.java
 
-common_templates = gcp.CommonTemplates()
-templates = common_templates.java_library()
-s.copy(templates, excludes=[
+java.common_templates(excludes=[
   'README.md',
   '.kokoro/presubmit/integration.cfg'
 ])