Allow java targets to support 1.8 source / target

This change enables build rules to specify:

LOCAL_JAVA_LANGUAGE_VERSION := 1.8

to enable -source 1.8 -target 1.8 for javac and
equivalent flags for Jack.

Bug: 26753820
(cherry-picked from commit cdfbe4a8522b9a13ffe6ac1f826a1d6760f87d60)

Change-Id: I361c99dd599e7b4a041f02c9562e461da2b0502e
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 49e56c4..c431792 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -337,6 +337,7 @@
 LOCAL_CLANG_64:=
 LOCAL_INIT_RC_32:=
 LOCAL_INIT_RC_64:=
+LOCAL_JAVA_LANGUAGE_VERSION:=
 
 # Trim MAKEFILE_LIST so that $(call my-dir) doesn't need to
 # iterate over thousands of entries every time.