Fix missing keyboard layouts for Linux/Trusty

Some keyboards chosen by the user never showed up in WeSay, while
other keyboards that were not chosen did show up.  Who knew that
Gnome uses country codes for primary keys in keyboard identifiers
instead of language codes?  This fixes WS-148 (and almost certainly
FWNX-1439).
(cherry picked from commit 51ee1548086ec017a15a23101a31b275741bf055)
All keyboards should use CombinedKeyboardAdaptor on Trusty

The default keyboard was still trying to use XkbKeyboardAdaptor, which
doesn't work on Trusty.  Thus, for example, the keyboard would never
switch back to the English keyboard after switching to the vernacular
language's keyboard in WeSay.  The same would probably happen in FW.
(cherry picked from commit be92b263df459655f8a5d3eee6dcd0023fb60f58)