Define vndk.private property for VNDK-private libraries

To define VNDK-private libraries, we used `vendor_available: false`.
Because of it, `vendor_available == nil` had different meaning from
`vendor_available: false` for the VNDK libraries.
To clarify this, we change the logic for defining VNDK-private
libraries which was:

cc_library {
    name: "vndk_private",
    vendor_available: false,
    product_available: false,
    vndk: {
        enabled: true,
    },
}

It must be replaced with

cc_library {
    name: "vndk_private",
    vendor_available: true,
    product_available: true,
    vndk: {
        enabled: true,
        private: true,
    },
}

Bug: 175768895
Test: m nothing
Change-Id: I81769f57c2231e54b682a28e4b82631ab9f3d390
diff --git a/cc/image.go b/cc/image.go
index b55e1f4..13d77cc 100644
--- a/cc/image.go
+++ b/cc/image.go
@@ -245,13 +245,9 @@
 						"vendor_available must be set to either true or false when `vndk: {enabled: true}`")
 				}
 				if m.VendorProperties.Product_available != nil {
-					// If product_available is defined for a VNDK, make sure vendor_available and
-					// product_available has the same value since `false` for these properties
-					// means the module is VNDK-private.
-					if Bool(m.VendorProperties.Vendor_available) != Bool(m.VendorProperties.Product_available) {
-						mctx.PropertyErrorf("product_available", "may not have different value than `vendor_available` for a VNDK")
-					}
-					// Also, both variants must have the same properties since they share a single VNDK library on runtime.
+					// If a VNDK module creates both product and vendor variants, they
+					// must have the same properties since they share a single VNDK
+					// library on runtime.
 					if !m.compareVendorAndProductProps() {
 						mctx.ModuleErrorf("product properties must have the same values with the vendor properties for VNDK modules")
 					}