Move IRemoteInputConnectionInvoker to android.inputmethodservice

IRemoteInputConnectionInvoker is used only from RemoteInputConnection,
which is already placed under android.inputmethodservice as a
package-private class.  Let's also move IRemoteInputConnectionInvoker
to android.inputmethodservice as a package-private class to minimize
its visibility.

This is a mechanical refactoring, which should have no user/developer
observable behavior change.

Bug: 234882948
Test: presubmit
Change-Id: Ie946993aeedc31a67e3074dc591245f75143a9f1