Plumb IME subtype change from IMMS to IMS.

This is a plumbing CL from IMMS to IMS to notify when the current input
method subtype is changed.  Those events are supposed to be used to
change physical keyboard layout depending on input method subtype, which
is to be implemented in subsequent CLs.

Bug: 25753054
Change-Id: I58e71ffce9ac9131551a00dd35e24235dadfef87