Add product_available

Vendors already can use these targets on /vendor. This change makes them
available to /product as well.

More about product partitions:
https://source.android.com/docs/core/architecture/partitions/product-partitions

Bug: 336989719
Test: mm
Change-Id: I30254a178d171810b5e12febbc712648331d31ba
diff --git a/Android.bp b/Android.bp
index 73f3ec1..7f314cf 100644
--- a/Android.bp
+++ b/Android.bp
@@ -651,6 +651,7 @@
     name: "grpc_deps_defaults",
     host_supported: true,
     vendor_available: true,
+    product_available: true,
     apex_available: [
         "//apex_available:platform",
     ],