Add LOCAL_MIN_SDK_VERSION to mk

For additional details see:

https://groups.google.com/a/google.com/d/msg/android-eng/PinDAM6WiHA/SAnSts84BAAJ

Change-Id: I78d01decc27bd470098a25aef3c9fe3f242e7334
diff --git a/tests/Android.mk b/tests/Android.mk
index fe4c9b2..4fd947c 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -18,5 +18,6 @@
 LOCAL_INSTRUMENTATION_FOR := Contacts
 
 LOCAL_SDK_VERSION := current
+LOCAL_MIN_SDK_VERSION := 21
 
 include $(BUILD_PACKAGE)