Add "product_available" to product available modules

"vendor_available" modules were available to product modules.
However, not all "vendor_available" modules are required to be
available to product modules. Some modules want to be available only
to product modules but not vendor modules.

To cover the requirement, we separate "product_available" from
"vendor_available".
"vendor_available" will not provide product available module.

Bug: 150902910
Test: build
Change-Id: I13b8222e191333c6c2cb7794ef3344fdcc6ebe98
diff --git a/libc/async_safe/Android.bp b/libc/async_safe/Android.bp
index 90cff0d..66389b9 100644
--- a/libc/async_safe/Android.bp
+++ b/libc/async_safe/Android.bp
@@ -9,6 +9,7 @@
 
     name: "libasync_safe",
     vendor_available: true,
+    product_available: true,
     recovery_available: true,
     native_bridge_supported: true,