Set JAVAC_WRAPPER if USE_GOMA is set.

Now, people can also use goma for building javac.  Let me make it by
default.

Test: USE_GOMA=true m -j4
Bug: 62334576
Change-Id: I941ce42dc0f9cbd0dcc00e6fd19368c3af11695f
diff --git a/core/config.mk b/core/config.mk
index 5ff582a..5895128 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -308,6 +308,7 @@
 
 export CC_WRAPPER
 export CXX_WRAPPER
+export JAVAC_WRAPPER
 endif
 
 ifdef TARGET_PREFER_32_BIT