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/Android.bp b/Android.bp
index 2af7ca4..ffc1df4 100644
--- a/Android.bp
+++ b/Android.bp
@@ -458,7 +458,8 @@
     installable: false, // this lib is a build-only library
     static_libs: [
         "framework-minus-apex",
-        // TODO(jiyong): add stubs for APEXes here
+        "framework-sdkext-stubs-systemapi",
+        // TODO(jiyong): add more stubs for APEXes here
     ],
     sdk_version: "core_platform",
 }