Clean up Javac a little bit.

- Added GLOBAL_JAVAC_DEBUG_FLAGS and merge it to
  PRIVATE_JAVACFLAGS/PRIVATE_JACK_FLAGS to get rid of
  PRIVATE_JAVAC_DEBUG_FLAGS/PRIVATE_JACK_DEBUG_FLAGS.
- With Java rules out of base_rules.mk we can get rid
  of java_alternative_checked_module now.

Change-Id: I1a14716c785e3d49330a75044107662ce96a9307
diff --git a/core/combo/javac.mk b/core/combo/javac.mk
index e3838a6..d00f541 100644
--- a/core/combo/javac.mk
+++ b/core/combo/javac.mk
@@ -35,6 +35,8 @@
     $(info CUSTOM_JAVA_COMPILER=eclipse)
 endif
 
+GLOBAL_JAVAC_DEBUG_FLAGS := -g
+
 HOST_JAVAC ?= $(COMMON_JAVAC)
 TARGET_JAVAC ?= $(COMMON_JAVAC)