Allow to force min sdk given to Jack

Allows setting the min sdk version for target Jack commands,
overriding the default value obtained from LOCAL_SDK_CURRENT.

Bug: 28188420
Change-Id: I1b84ad8a7b6ae30639f8ade455c1e542af5692e4
diff --git a/core/java.mk b/core/java.mk
index 5ac9342..3f1966d 100644
--- a/core/java.mk
+++ b/core/java.mk
@@ -51,6 +51,10 @@
   endif
 endif
 
+ifneq (,$(strip $(LOCAL_MIN_SDK_VERSION)))
+  my_jack_min_sdk_version := $(LOCAL_MIN_SDK_VERSION)
+endif
+
 proto_sources := $(filter %.proto,$(LOCAL_SRC_FILES))
 ifneq ($(proto_sources),)
 ifeq ($(LOCAL_PROTOC_OPTIMIZE_TYPE),micro)