Add utility of enabled InputMethod settings to InputMethodManagerService

- Add setter of enabled InputMethodSubtypes
- Enabled InputMethods are saved as follows:
-- enabled_inputmethod0;enabled_subtype0;enabled_subtype1:enabled_inputmethod1
- TODO: Fix Settings application to parse new Enabled InputMethod string.
-- Currently IMMS doesn't save InputMethodSubtypes so this will not break Settings application.

Change-Id: Icd0f13de396ce286ff6563e8c2775d53bcdacbf3