[components] Closes https://github.com/mozilla-mobile/android-components/issues/8693: Integrate GeckoWebExecutor#FETCH_FLAG_PRIVATE.

mobile/android/android-components/components/lib/fetch-httpurlconnection/src/main/java/mozilla/components/lib/fetch/httpurlconnection/HttpURLConnectionClient.kt

@@ -32,6 +32,9 @@ class HttpURLConnectionClient : Client() {
 
     @Throws(IOException::class)
     override fun fetch(request: Request): Response {
+        if (request.private) {
+            throw IllegalArgumentException("Client doesn't support private request")
+        }
         if (request.isDataUri()) {
             return fetchDataUri(request)
         }