Define product_available property

To make a module available to product variants, it must define
`product_available: true`. `vendor_available: true` will not create
product variants any more.
However, in this CL, we don't change the behavior of
`vendor_available` property. It still creates both variants. After we
update all Android.bp files that need to provide product variants
with `product_available: true`, we may upload the remaining patches.

Bug: 150902910
Test: lunch aosp_arm64-userdebug && m
Change-Id: I0fd5be7bbae2c45d5cab3c3c2ca49f53a9b6f975

Android.bp

@@ -46,6 +46,7 @@ toolchain_library {
     name: "libatomic",
     defaults: ["linux_bionic_supported"],
     vendor_available: true,
+    product_available: true,
     ramdisk_available: true,
     vendor_ramdisk_available: true,
     recovery_available: true,
@@ -71,6 +72,7 @@ toolchain_library {
     name: "libgcc",
     defaults: ["linux_bionic_supported"],
     vendor_available: true,
+    product_available: true,
     recovery_available: true,
     native_bridge_supported: true,
 
@@ -94,6 +96,7 @@ toolchain_library {
     name: "libgcc_stripped",
     defaults: ["linux_bionic_supported"],
     vendor_available: true,
+    product_available: true,
     ramdisk_available: true,
     vendor_ramdisk_available: true,
     recovery_available: true,