Make nist-sip compile against public SDK APIs

nist-sip is the only dependency of the ext.jar that
wasn't explicitly building against public SDK
APIs. This change enables us to enforce use of
public SDKs in Android's ext.jar.

Bug: 113148576
Test: make ext
Change-Id: I7d38d35f9069f60af68b64e3bac816476780b39c
diff --git a/Android.bp b/Android.bp
index 81afdc9..46a4cc9 100644
--- a/Android.bp
+++ b/Android.bp
@@ -15,7 +15,7 @@
 java_library_static {
     name: "nist-sip",
     srcs: ["java/**/*.java"],
-    no_framework_libs: true,
+    sdk_version: "core_current",
     errorprone: {
         javacflags: [
             "-Xep:MissingOverride:OFF",  // Ignore missing @Override.