Make ext.jar only dependent on public SDK APIs

ext.jar can be built using only public SDK APIs. This
makes that obvious.

Bug: 113148576
Test: make ext
Change-Id: I792b14924878623f832f3afbb1d3e23fba34d5bf
diff --git a/Android.bp b/Android.bp
index b2b3dff..d1ae3b7 100644
--- a/Android.bp
+++ b/Android.bp
@@ -795,7 +795,7 @@
 java_library {
     name: "ext",
     installable: true,
-    no_framework_libs: true,
+    sdk_version: "core_current",
     static_libs: [
         "libphonenumber-platform",
         "nist-sip",