We (Growth team) would like to use the language overlay for selecting a language
in our welcome survey to new users. (See T232410.) The existing language overlay
is built to load language links for the current page, but it is easily
adapted for use with obtaining language info.

To do this, this patch:

* Adds a languageInfoOverlay
* Adds a LanguageInfo class for obtaining information via the API, with some
light-weight manipulation of the data so it's presentable in the overlay. A nice
improvement would be to make use of the UniversalLanguageExtension data to
provide additional ways to search (e.g. "Spanish" in addition to
español); a comment explaining a possible approach to this is in the patch
* Fires an event when a language is picked via LanguageSearcher

Because of the additional code, the mobile.startup.js size is now 5.34KB. It's
possible to lower this below the previous threshold of 5.2KB by making
LanguageOverlay do more, but the code becomes less clear by doing so.

Bug: T232410
Change-Id: Ic0f1613e8fd27788f67ed6235386491f0fe077c3