Check LEGACY_USE_JAVA7 before using 1.8.

Fixes current bots not updated to Java8 yet.

Change-Id: I61f5a3f0de6ca0ee9eca64f0ee08454f93e59704
diff --git a/core/java_common.mk b/core/java_common.mk
index 1b9a861..d9ce9df 100644
--- a/core/java_common.mk
+++ b/core/java_common.mk
@@ -4,7 +4,11 @@
 ## Java version
 ###########################################################
 ifeq (,$(LOCAL_JAVA_LANGUAGE_VERSION))
-  LOCAL_JAVA_LANGUAGE_VERSION := 1.8
+  ifeq (,$(LEGACY_USE_JAVA7))
+    LOCAL_JAVA_LANGUAGE_VERSION := 1.8
+  else
+    LOCAL_JAVA_LANGUAGE_VERSION := 1.7
+  endif
 endif
 LOCAL_JAVACFLAGS += -source $(LOCAL_JAVA_LANGUAGE_VERSION) -target $(LOCAL_JAVA_LANGUAGE_VERSION)