Use vndk.private: true for VNDK-private libs

Instead of using `vendor_available: false`, use `vndk.private: true`
for VNDK-private libraries.

Bug: 175768895
Test: build
Change-Id: Ia9c38488e41ac59272ba573fe4d7b262408821d5
diff --git a/Android.bp b/Android.bp
index a27ced2..86f15b0 100644
--- a/Android.bp
+++ b/Android.bp
@@ -168,12 +168,13 @@
 
 cc_library_shared {
     name: "libblas",
-    vendor_available: false,
+    vendor_available: true,
     // TODO(b/153609531): remove when no longer needed.
     native_bridge_supported: true,
     vndk: {
         enabled: true,
         support_system_process: true,
+        private: true,
     },
     defaults: ["cblas-defaults"],
     static_libs: ["libF77blas"],