Provide make variables for java binaries

Replace java -> $(JAVA), etc. to prepare for moving java binaries
out of the path and using full paths to them.

Test: builds
Bug: 62956999
Change-Id: I5dfcd0d4e855ecde69cd127169e9545e59723d12
diff --git a/core/combo/javac.mk b/core/combo/javac.mk
index 7f66ea8..99bd0ee 100644
--- a/core/combo/javac.mk
+++ b/core/combo/javac.mk
@@ -23,6 +23,10 @@
 JAVACC := $(ALTERNATE_JAVAC)
 endif
 
+JAVA := java
+JAVADOC := javadoc
+JAR := jar
+
 # The actual compiler can be wrapped by setting the JAVAC_WRAPPER var.
 ifdef JAVAC_WRAPPER
     ifneq ($(JAVAC_WRAPPER),$(firstword $(JAVACC)))