Annotate threads for some IME handling methods

This is a preparation to work on Bug 36897707.

For instance, the reason why most of IME-related callbacks in
InputMethodService get called on the main thread is because
IInputMethodWrapper keeps forwarding incoming IPCs into the
main looper of the IME process as follows:

  InputMethodManagerService (IMMS)
        ------
    -> one-way binder IPCs over	IInputMethod
        ------
      -> IInputMethodWrapper (on the binder thread(s))
       -> Handler (to dispatch tasks to main thread)
        -> InputMethodImpl.* (on the main thread)
         -> InputMethodService.* (on the main thread)

By adding explicit annotations such as @BinderThread and @MainThread
in relevant methods, this CL makes that kind of investigation much
easier than before.

Bug: 36897707
Test: compile
Change-Id: I8f9afe9a1986a9fa41fb66fdc64e8f0f67e45c2e