[Topics] Refactor header to be easier to read and parse

Changed the format of the topics request header from:
"t=(1;v=chrome.1:1:2, 2), p=P00000" to
"(1 2);v=chrome.1:1:2, ();p=P00000"

As discussed in patcg-individual-drafts/topics#183

Bug: 1443540
Change-Id: I3ea40df9f8732f5eff953110f22f4da2163480fb

browsing-topics/resources/header-util.sub.js

@@ -1,4 +1,4 @@
-const EMPTY_TOPICS_HEADER = 't=(), p=P000000000000000000000000000';
+const EMPTY_TOPICS_HEADER = '();p=P00000000000000000000000000000';
 
 function test_topics_iframe_navigation_header(
     test, has_browsing_topics_attribute, is_same_origin, expect_topics_header_available_func) {