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

cc/library_sdk_member.go

@@ -130,6 +130,10 @@ func (mt *librarySdkMemberType) AddPrebuiltModule(ctx android.SdkMemberContext,
 		pbm.AddProperty("vendor_available", true)
 	}
 
+	if proptools.Bool(ccModule.VendorProperties.Product_available) {
+		pbm.AddProperty("product_available", true)
+	}
+
 	sdkVersion := ccModule.SdkVersion()
 	if sdkVersion != "" {
 		pbm.AddProperty("sdk_version", sdkVersion)