Always specify non-null Locale object to SuggestionSpan

Confusingly, specifying a null Locale object to the constructor
of SuggestionSpan does not necessarily mean that
SuggestionSpan#getLocale() returns null.  The constructor in
question also receives Context object, and Context's locale can
be used as a fallback locale to initialize locale of
SuggestionSpan.

With this CL, LatinIME always specify non-null Locale object
when instantiating SuggestionSpan object.  It basically
corresponds to the active main dictionary, but can be
Locale#ROOT when one locale is not determined for some reasons.

BUG: 20435013
Change-Id: I2c152466410327300e7dba4d7ed9a22f57c17c4f