Deprecate InputMethodManager#setAdditionalInputMethodSubtypes()

This CL marks InputMethodManager#setAdditionalInputMethodSubtypes()
deprecated.  There is no actual implementation change at this moment,
but developers should be informed that this API may stop working in a
future version of Android.  IME developers should start migration
right now.

Since this feature was implemented in Android 4.0 ICS [1], there have
been many tricky bugs and documented/undocumented pitfalls.  For IMEs
that have already implemented features like customizable/downloadable
keyboard layouts/languages, most likely re-implementing IMEs' own
language switching mechanism then exposing only one unified IME
subtype to the system would be a better long term choice.

 [1]: Ic7869cfaa5361531e08d58d7dfa5ba0feab0613e
      ee5e77ca

Fix: 121220151
Test: make -j checkbuild
Change-Id: I781abde2d6d97b8420b29b80e14b9cce97482754