Make sure to compare subtype language with system language.

This CL addresses a regression introduced by a recent CL [1] that
non-keyboard subtypes are no longer implicitly enabled based on the
system language (a.k.a. "use system language" in the subtype enabler)
due to a type mismatch in comparison.

Here is the original logic:
 if (language.equals(systemLanguage) && systemLocale.startsWith(locale))

And here is the logic replaced by [1]:
 if (locale != null && locale.equals(systemLanguage)) {

The new logic is simply broken, because locale is a Locale object while
systemLanguage is a String object.  It never matches.

With this CL we will compare the system language with the locale
language again, with several test cases that should have been included
in [1], as a temporary solution until we start relying on
LocaleUtils#filterByLanguage() for non-keyboard subtypes.

  [1]: Iaf179d60c12b9a98b4f097e2449471c4184e049b
       e985c240

Bug: 27560993
Change-Id: If2d1710174853180465832e6ecbbb91235b76210