rust: Emit android vndk cfg flag.

Pass an "android_vndk" cfg flag that indicates this code
targets the vndk. This can be useful in instances where code
might need to behave differently.

This also includes a fix to make sure our vendor Soong tests
are correctly configured.

Bug: 179907868
Test: Soong tests pass.
Test: Example module emits new cfg flags.
Change-Id: I01cdf91f6f9d42cd8a759266d5170479664bf4bc

rust/compiler.go

@@ -232,6 +232,10 @@ func (compiler *baseCompiler) compilerFlags(ctx ModuleContext, flags Flags) Flag
 		flags.LinkFlags = append(flags.LinkFlags, "-Wl,-rpath,"+rpathPrefix+"../"+rpath)
 	}
 
+	if ctx.RustModule().UseVndk() {
+		flags.RustFlags = append(flags.RustFlags, "--cfg 'android_vndk'")
+	}
+
 	return flags
 }