Do not read 'vendor_available: false'

In case of VNDK, 'vendor_available: false' had a special meaning that
defines VNDK-private libraries. It is not trivial because not
defining a boolean property means 'false' normally. To avoid the
confusion replace it with the 'vndk.private: true' for VNDK-private
libraries and 'private: true' for LLNDK-private libraries.

All VNDK libraries must define 'vendor_available: true' and may have
'vndk.private: true' if they are VNDK-private.
With this change '(vendor|product)_available: false' is the same as
not defining the property.

LLNDK-private must define 'private: true' instead of
'vendor_available: false'.

Bug: 175768895
Test: build
Change-Id: I57fbca351be317257d95027f3cdcdbbe537eab23

cc/testing.go

@@ -299,7 +299,7 @@ func GatherRequiredDepsForTest(oses ...android.OsType) string {
 		llndk_library {
 			name: "libft2.llndk",
 			symbol_file: "",
-			vendor_available: false,
+			private: true,
 			sdk_version: "current",
 		}
 		cc_library {