[YouTube] Fix page reload required error on streams

Playability status for the WEB client isn't checked anymore, as it
requires a valid signatureTimestamp from JavaScript player to avoid a
page reload error. Until we can get and use n parameter function
decoding for streaming URLs of HTML5 clients again and support SABR
streams, it doesn't really make sense to fetch a huge JavaScript player
just to get metadata.

Age-restricted content which can be watched with the embedded player is
also broken due to extraction failure and non-ability to run the n
parameter function, so fetching this embedded player has been also
removed and all age-restricted content will throw an
AgeRestrictedContentException.

Playability error status checks have been adapted, as mobile clients get
different error responses than the WEB client.

Some strings have been extracted into constants and a fallback for
unlisted privacy check has been added, in YoutubeStreamExtractor for
both points.

Author: AudricV

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeStreamHelper.java

@@ -72,7 +72,7 @@ public static JsonObject getWebMetadataPlayerResponse(
                 .getBytes(StandardCharsets.UTF_8);
 
         final String url = YOUTUBEI_V1_URL + PLAYER + "?" + DISABLE_PRETTY_PRINT_PARAMETER
-                + "&$fields=microformat,playabilityStatus,storyboards,videoDetails";
+                + "&$fields=microformat,videoDetails.thumbnail.thumbnails,videoDetails.videoId";
 
         return JsonUtils.toJsonObject(getValidJsonResponseBody(
                 getDownloader().postWithContentTypeJson(