Always use Slog in InputMethodSubtypeSwitchingController

Mixing android.util.Log and android.util.Slog doesn't make much sense.
Let's always android.util.Slog here for consistency.

There should be no observable behavior change as the logging code in
question is used only when InputMethodSubtypeSwitchingController#DEBUG
is set to "true" before rebuilding the system image.

Bug: 171827834
Test: presubmit
Test: atest FrameworksServicesTests:InputMethodSubtypeSwitchingControllerTest
Change-Id: I9f2dce3447051981cc30a00525ecf37f8e588ee9