Use IndexablePresence for hasFileAttachments and hasVisualMediaAttachments

Reduces index size, makes it easier to debug using IndexedDB inspector, and
hopefully improves lookup performance.

test/backup_test.js

@@ -350,7 +350,7 @@ describe('Backup', () => {
             ]).buffer,
           }],
           hasAttachments: 1,
-          hasFileAttachments: 0,
+          hasFileAttachments: undefined,
           hasVisualMediaAttachments: 1,
           quote: {
             text: "Isn't it cute?",

test/modules/types/message_test.js

@@ -182,7 +182,7 @@ describe('Message', () => {
           size: 1111,
         }],
         hasAttachments: 1,
-        hasVisualMediaAttachments: 0,
+        hasVisualMediaAttachments: undefined,
         hasFileAttachments: 1,
         schemaVersion: Message.CURRENT_SCHEMA_VERSION,
       };