Simplify IInputMethod#startInput() handler a bit

This is a follow up CL to our previous CL [1], which introduced a
nested SomeArgs to pass more integer parameters while handling
IInputMethod#startInput() callback.

Now that only 2 integers and 4 objects are required to handle
IInputMethod#startInput(), single SomeArgs should be good enough to
pack everything into a Message object.

This CL does nothing other than applying such a mechanical code clean
up.  There should be no observable behavior difference.

 [1]: I1a6300fe167eb205ee2b4214a6e270a52ebae062
      eadb1392

Bug: 192412909
Bug: 194110780
Test: presubmit
Change-Id: I90eff3bfc5ed0f43472b0cd49aa532474a60c266