Changing to build with system API

Since the Dialer is the system dialer on Nexus devices pre-M, we need
access to some system apis for backporting.

Bug: 25776171
Change-Id: I4365d4df955eb3f3ee7889937a97d1201e205d12
diff --git a/Android.mk b/Android.mk
index bd0c9de..47648db 100644
--- a/Android.mk
+++ b/Android.mk
@@ -60,7 +60,7 @@
 
 # Uncomment the following line to build against the current SDK
 # This is required for building an unbundled app.
-LOCAL_SDK_VERSION := current
+LOCAL_SDK_VERSION := system_current
 
 include $(BUILD_PACKAGE)