Use "languageTag" in RichInputMethodSubtype.

With this CL, RichInputMethodSubtype#getLocale() starts returning
a Locale object that is initialized with "languageTag" when it is
specified.  No behavior change is intended when "languageTag" attribute
is not available or specified.

Bug: 22858221
Change-Id: I23f2e479b8e284ce589c6950b071ba84c5dd8ce1