Handle cancellation of 3rd party IME explicitly.

When a 3rd party IME is enabled for the first time, a warning dialog is
shown.
The dialog has a cancel button, and it triggers
onSaveInputMethodPreference(). However, when it's cancelled by the back
button, it did not trigger onSaveInputMethodPreference().

It still works as intended on stock Android, but on ARC++ we need the
callback to be always fired every time it's cancelled.

Test: manual
Bug: 112442972
Change-Id: Ibdbaa020957ccd5a77a7fe5c67937edd19b8455f