Pin support libraries to Java 1.7 so they can be used with dx

The .jar artifacts must contain v51 class files to prevent them
being rejected by dx.

Bug: 26753820
Bug: 27353172
Bug: 27338966
Change-Id: I03a881a86bb6e3fcaa4ccb33e6c0615157ee363f
diff --git a/v7/palette/Android.mk b/v7/palette/Android.mk
index 5c9898f..0c4cb05 100644
--- a/v7/palette/Android.mk
+++ b/v7/palette/Android.mk
@@ -21,6 +21,7 @@
 LOCAL_SRC_FILES := $(call all-java-files-under, src/main)
 LOCAL_MANIFEST_FILE := $(LOCAL_PATH)/src/main/AndroidManifest.xml
 LOCAL_JAVA_LIBRARIES += android-support-v4
+LOCAL_JAVA_LANGUAGE_VERSION := 1.7
 include $(BUILD_STATIC_JAVA_LIBRARY)
 
 # API Check