Add SdkExtensions class

It currently just reads a prop that is never set, but that will
be updated separately.

Keep the class @hide for now.

Bug: 137191822
Test: m com.android.sdkext
Change-Id: I1a874bf3d3e34676b5d767c4d03b09337b193cce
diff --git a/apex/sdkext/Android.bp b/apex/sdkext/Android.bp
index 60d657d..b8dcb90 100644
--- a/apex/sdkext/Android.bp
+++ b/apex/sdkext/Android.bp
@@ -15,6 +15,7 @@
 apex {
     name: "com.android.sdkext",
     manifest: "manifest.json",
+    java_libs: [ "framework-sdkext" ],
     key: "com.android.sdkext.key",
     certificate: ":com.android.sdkext.certificate",
 }