Bug 835350 - Bookmarks customization depending on MCC SIM card

reuse getICCSettings pattern from system/js/operator_variant/operator_variant.js

add zfill function to pad leading zeros for mcc/mnc, now always 6 digits

Use DEFAULT_BOOKMARK, DEFAULT_MNC instead of spread strings
Merge pull request #8606 from gasolin/issue-835350

Issue 835350 Customize Browser Bookmark via iccsettings, r=bfrancis