Minimize the number of default enabled IMEs part 3

With this CL, the behavior of getDefaultEnabledImes() changes
as follows:

- Previously system IMEs are always enabled by default as long
  as it is a software keyboard that supports En_* subtype. With
  this CL, getDefaultEnabledImes() relies on the locale returned
  from getFallbackLocaleForDefaultIme() instead of calling
  isSystemImeThatHasEnglishKeyboardSubtype() to minimize the
  number of enabled IMEs.
- Previously default enabled system IMEs are chosen in a
  country-agnostic way. As a result, "en_IN" is enabled even
  when the system locale is "en_US". With this CL, the system
  first tries to find IMEs with taking the coutnry into account,
  and use the country-agnostic way when and only when fallback
  logic is required.

BUG: 17347871
Change-Id: I6571d464a46453934f0a8f5e79018a67a9a3c845