Force javac to use -encoding UTF-8

Since I3778f9c8 we have had non-ASCII characters in our source files.
javac may decide to read the locale from its environment, which fails
on such source files, causing warnings at best and test failures at
worst.

Hack our default java rule defs to pass -encoding.

Change-Id: I53dfd2d551eed7c1c1fa8ef5cfcfb726d67e3a62

tools/default.defs

@@ -20,18 +20,38 @@ include_defs('//tools/java_doc.defs')
 include_defs('//tools/java_sources.defs')
 import copy
 
-# Add AutoValue support to java_library.
+# Set defaults on java rules:
+#  - Add AutoValue annotation processing support.
+#  - Treat source files as UTF-8.
+
 _buck_java_library = java_library
 def java_library(*args, **kwargs):
-  _set_auto_value(kwargs)
+  _munge_args(kwargs)
   _buck_java_library(*args, **kwargs)
 
-# Add AutoValue support to java_test.
 _buck_java_test = java_test
 def java_test(*args, **kwargs):
-  _set_auto_value(kwargs)
+  _munge_args(kwargs)
   _buck_java_test(*args, **kwargs)
 
+
+# Munge kwargs to set Gerrit-specific defaults.
+def _munge_args(kwargs):
+  _set_auto_value(kwargs)
+  _set_extra_arguments(kwargs)
+
+def _set_extra_arguments(kwargs):
+  ext = 'extra_arguments'
+  if ext not in kwargs:
+    kwargs[ext] = []
+  extra_args = kwargs[ext]
+
+  for arg in extra_args:
+    if arg.startswith('-encoding'):
+      return
+
+  extra_args.extend(['-encoding', 'UTF-8'])
+
 def _set_auto_value(kwargs):
   apk = 'annotation_processors'
   if apk not in kwargs:
@@ -47,6 +67,7 @@ def _set_auto_value(kwargs):
     aps.extend(AUTO_VALUE_PROCESSORS)
     apds.extend(AUTO_VALUE_PROCESSOR_DEPS)
 
+
 def genantlr(
     name,
     srcs,