Annotation - For checkboxes, get field value from AS (if any) instead of V (bug 1722036)

- it aims to fix https://bugzilla.mozilla.org/show_bug.cgi?id=1722036.
  - AS and V should share the same value for checkbox: it's at least what the specs say;
  - the pdf in the above bug opens correctly in Acrobat so it likely means that AS is chosen over V.

test/pdfs/bug1722036.pdf.link

@@ -0,0 +1 @@
+https://bugzilla.mozilla.org/attachment.cgi?id=9232845

test/test_manifest.json

@@ -5899,5 +5899,13 @@
        "rounds": 1,
        "lastPage": 1,
        "type": "eq"
+    },
+    {  "id": "bug1722036",
+       "file": "pdfs/bug1722036.pdf",
+       "md5": "bbc7f62ca85609c97069bd3ce2658288",
+       "rounds": 1,
+       "link": true,
+       "forms": true,
+       "type": "eq"
     }
 ]