Fix minor style issues in InputMethodService

This is a minor follow up CL to our previous CL [1], which migrated
IME to handle back with OnBackInvokedDispatcher.

Having SomeArgs within SomeArgs doesn't make sense unless we really
run out of arguments.

This CL also addresses minor style issues.

Basically this is a mechanical clean-up that has no behavior change.

 [1]: Ie207b63b11a56c9b2173f26b734a27b13ebccc60
      a6666f22

Bug: 228358882
Bug: 234882948
Test: presubmit
Change-Id: Ie14f66be0c9d66d221c5f9fae244df5487d65593