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