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/Android.bp b/Android.bp
index 63c8fb6..5efcb77 100644
--- a/Android.bp
+++ b/Android.bp
@@ -561,6 +561,12 @@
     ],
 }
 
+java_library {
+    name: "framework-annotations-lib",
+    srcs: [ ":framework-annotations" ],
+    sdk_version: "current",
+}
+
 filegroup {
     name: "framework-networkstack-shared-srcs",
     srcs: [