androidbp-impl emits proprietary: true

Test: generate project with c++-impl/anroidbp-impl, built correctly to
the vendor partition
Bug: 34135607

Change-Id: I8af6d148f64e6ae35e8133c8e5c4a0cc3b33664f
diff --git a/main.cpp b/main.cpp
index 7426e65..647ba1c 100644
--- a/main.cpp
+++ b/main.cpp
@@ -854,6 +854,7 @@
     out.indent([&] {
         out << "name: \"" << libraryName << "\",\n"
             << "relative_install_path: \"hw\",\n"
+            << "proprietary: true,\n"
             << "srcs: [\n";
         out.indent([&] {
             for (const auto &fqName : packageInterfaces) {