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/combo/javac.mk b/core/combo/javac.mk
index 70dae13..7f66ea8 100644
--- a/core/combo/javac.mk
+++ b/core/combo/javac.mk
@@ -14,7 +14,7 @@
 ANDROID_COMPILE_WITH_JACK := true
 endif
 
-common_jdk_flags := -source 1.7 -target 1.7 -Xmaxerrs 9999999
+common_jdk_flags := -Xmaxerrs 9999999
 
 # Use the indexer wrapper to index the codebase instead of the javac compiler
 ifeq ($(ALTERNATE_JAVAC),)