Remove extension monitor dependency on FUEL/STEEL/SMILE

[1ec5]

The about:addons overlay, which warns about Mudim and Typing Vietnamese being installed, depends on extIApplication.getExtensions(), part of FUEL in Firefox, aka STEEL in Thunderbird, aka SMILE in SeaMonkey. Firefox has removed FUEL in Nightly and Aurora builds, so we should instead import AddonManager.jsm and use AddonManager.getAddonByID().