Add targets for building individual modules using javac

This is needed in order to be able to run error prone (which is
a plugin replacement for javac) on a subset of the java code.

Bug: 31507496
Test: Build javac-check-core-tests and check the resulting JAR
Change-Id: I5a556920572fcdad7b39d67e3f00fe0e0f1f8f46
diff --git a/core/java.mk b/core/java.mk
index e505ffb..606b474 100644
--- a/core/java.mk
+++ b/core/java.mk
@@ -416,6 +416,7 @@
 	$(transform-java-to-classes.jar)
 
 javac-check : $(full_classes_compiled_jar)
+javac-check-$(LOCAL_MODULE) : $(full_classes_compiled_jar)
 
 # Run jarjar if necessary, otherwise just copy the file.
 ifneq ($(strip $(LOCAL_JARJAR_RULES)),)