Initialize InputMethodSubtype with "languageTag".

My previous commit [1] introduced a new XML attribute "languageTag" for
subtypes but forgot to initialize InputMethodSubtype object with that
attribute.  As a result, InputMethodSubtype#getLanguageTag() has always
returned null even if "languageTag" attribute is specified.

  [1]: I77db5b99a7cf745d800db75baf135bb60ad04820
       8d6eeb01

With this CL, InputMethodSubtype#getLanguageTag() starts returning the
value specified in the XML resource.

Bug: 22859862
Change-Id: I251d3d999afd13c0d618f2cb59e8ed3d47f21c98