Allow ImeSutype with null languagtag to be passed to IMS

Imesubtypes with null languagtag should be allowed to be set.
If ImeSubtype is null, user can still select a PK layout for it
in the settings and we need the callbacks so that when user
switches to that particular ImeSubtype, we select the user
selected layout.

Test: atest InputMethodSubtypeTest
Bug: 20805588
Change-Id: I8541e6132bdf04245831010f11ceabb651dd34ca