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/droiddoc.mk b/core/droiddoc.mk
index eb12630..48a930d 100644
--- a/core/droiddoc.mk
+++ b/core/droiddoc.mk
@@ -179,7 +179,7 @@
 	$(call prepare-doc-source-list,$(PRIVATE_SRC_LIST_FILE),$(PRIVATE_JAVA_FILES), \
 			$(PRIVATE_SOURCE_INTERMEDIATES_DIR) $(PRIVATE_ADDITIONAL_JAVA_DIR))
 	$(hide) ( \
-		javadoc \
+		$(JAVADOC) \
                 -encoding UTF-8 \
                 -source 1.8 \
                 \@$(PRIVATE_SRC_LIST_FILE) \
@@ -216,7 +216,7 @@
 	$(call prepare-doc-source-list,$(PRIVATE_SRC_LIST_FILE),$(PRIVATE_JAVA_FILES), \
 			$(PRIVATE_SOURCE_INTERMEDIATES_DIR) $(PRIVATE_ADDITIONAL_JAVA_DIR))
 	$(hide) ( \
-		javadoc \
+		$(JAVADOC) \
                 -encoding UTF-8 \
                 $(PRIVATE_DROIDDOC_OPTIONS) \
                 \@$(PRIVATE_SRC_LIST_FILE) \