Add methods for managing subtypes

- added showInputMethodSubtypePicker to public API
-- show the selector dialog for subtypes
- added getter, setter and event handler to InputMethodManagerService
- extract InputMethodSubtype to the top level class for using it in aidl
- TODO: make an enabler for input method subtypes
- TODO: handle the event of changing an input method subtype in LatinIME

Change-Id: I49f8c6675ac4b06511635d14a37bd398738eff33