Add new switchToInputMethod API

Make accessibility service able to set the current input method
for current user.

Bug: 145710994
Test: atest AccessibilitySoftKeyboardTest
   1. Get current enabled IMEs
   $ adb shell settings get secure enabled_input_methods
   com.google.android.googlequicksearchbox/com.google.android.voicesearch.ime.VoiceInputMethodService
   com.google.android.inputmethod.latin/com.android.inputmethod.latin.LatinIME
   2. Get current default IME
   $ adb shell settings get secure default_input_method
   3. Set current default IME
   $ adb shell settings put secure default_input_method
     com.google.android.googlequicksearchbox/com.google.android.voicesearch.ime.VoiceInputMethodService
Change-Id: I00aa2170ef2fcc4daf50d09a57b8e2ad16c0a6cb