Add stubs and an sdk definition for sdkext

This defines both publicapi and a systemapi variants, even though
the publicapi is currently empty. Uses the newly defined module
stub_defaults.

Bug: 137191822
Test: m framework
Test: m sdkext-sdk
Change-Id: Idad8902054bef4a08953f4677689506566f146d8
diff --git a/apex/sdkext/Android.bp b/apex/sdkext/Android.bp
index aaf25b1..6b52481 100644
--- a/apex/sdkext/Android.bp
+++ b/apex/sdkext/Android.bp
@@ -25,6 +25,11 @@
     certificate: ":com.android.sdkext.certificate",
 }
 
+sdk {
+    name: "sdkext-sdk",
+    java_libs: [ "framework-sdkext-stubs-systemapi" ],
+}
+
 apex_key {
     name: "com.android.sdkext.key",
     public_key: "com.android.sdkext.avbpubkey",