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

apex/vndk_test.go

@@ -25,6 +25,7 @@ func TestVndkApexForVndkLite(t *testing.T) {
 			name: "libvndk",
 			srcs: ["mylib.cpp"],
 			vendor_available: true,
+			product_available: true,
 			vndk: {
 				enabled: true,
 			},
@@ -37,6 +38,7 @@ func TestVndkApexForVndkLite(t *testing.T) {
 			name: "libvndksp",
 			srcs: ["mylib.cpp"],
 			vendor_available: true,
+			product_available: true,
 			vndk: {
 				enabled: true,
 				support_system_process: true,
@@ -73,6 +75,7 @@ func TestVndkApexUsesVendorVariant(t *testing.T) {
 		cc_library {
 			name: "libfoo",
 			vendor_available: true,
+			product_available: true,
 			vndk: {
 				enabled: true,
 			},