Show toast when subtype is rotated by Meta-Space.

This follows up to my previous CL [1], with which we changed global
hardware key combinations regarding how multilingual user can manage
multiple keyboard layouts / input methods.  Now we offer Meta-Space to
rotate input method (subtype), while we offered Shift-Space to rotate
hardware keyboard layouts previously.

One thing that is lost during above transition is an indication when the
such a key combination takes effect.  Actually there was a toast that
shows new keyboard layout name, which is now lost.

With this CL, we bring back a toast so that we can show the new input
method (subtype) name. Note that the toast will be shown if all of the
following conditions are met.
 - The input method (subtype) is rotated by a hardware key combination.
 - IME window does not have IME_VISIBLE bit.  Otherwise, showing a toast
   is likely to cause UI overlap with the current IME's window.

  [1]: I4005692215edfcf8bed3e86b1e07000148f986f5
       ae61f711

Bug: 27547054
Change-Id: If3fe17adbffe4c1125783fc77fed3cfe78fc7933