Make update_engine_stable_client product_specific.

For its dependencies, use vendor_available to indicate it can be used by
core and product variants, because product_available is not available.

Test: builds
Bug: 161563386
Bug: 150902910

Change-Id: I6a951ee3ea91c945a830a32c0a117af4c73bb6b8
diff --git a/Android.bp b/Android.bp
index 008a432..9187e67 100644
--- a/Android.bp
+++ b/Android.bp
@@ -753,6 +753,13 @@
 // update_engine header library
 cc_library_headers {
     name: "libupdate_engine_headers",
+
+    // This header library is available to core and product modules.
+    // Right now, vendor_available is the only way to specify this.
+    // vendor modules should NOT use this library.
+    // TODO(b/150902910): change this to product_available.
+    vendor_available: true,
+
     export_include_dirs: ["."],
     apex_available: [
         "com.android.gki.*",