Make HTTPS Everywhere work on Firefox Mobile again

Firefox Mobile was supported on 4.0, and e10s was supported on master. When I merged the two, Mobile broke (all pages were blank white). This changes a bunch of places to use BrowserApp instead of gBrowser, which makes it mostly work again.

Still doesn't work perfectly. Notably, if you navigate from one page to the next, the list of applied rules isn't reset, so if you click the HTTPSE icon you'll get a list of rules that includes rules applied to previous pages. Working on that, going to submit as separate review.

cc @cooperq for code review.