Client-side reimplementation of IMM#setCurrentInputMethodSubtype()

This is a preparation to mark
InputMethodManager#setCurrentInputMethodSubtype() deprecated.

InputMethodManager#setCurrentInputMethodSubtype(), which was
introduced in Android 4.0 ICS [1], was probably mistakenly exposed as
a public API, because it has required WRITE_SECURE_SETTINGS that
typical applications cannot have.

Keeping maintaining InputMethodManager#setCurrentInputMethodSubtype()
is not that simple because now we are about to enable per-profile IME
mode, where this method needs to have a more clear spec about what
"Current" means.

An ideal solution is just removing this method, because if the caller
already has WRITE_SECURE_SETTINGS permission, they can just directly
update Settings.Secure.SELECTED_INPUT_METHOD_SUBTYPE to achieve the
same goal.  However, given that this has been a public API, it would
probably make sense to provide a fallback implementation like I did
for null IME token in InputMethodManager#setInputMethod() [2].

Either way, InputMethodManager#setCurrentInputMethodSubtype() will be
marked as deprecated in a subsequent CL.

 [1]: I55daa19ba924999def544bf841f00bf54852f3e1
      b66d287e
 [2]: I42dd0325b01c527009bf85566ca8ba0766b2294e
      0c1ebffd

Bug: 123249820
Test: manually done with a test app that has WRITE_SECURE_SETTINGS
Change-Id: I76da83c57cffc6b73defccfd4a1b5734c958a97e