Adjusting InputMethodManagerServiceWindowGainedFocusTest for refactor

Previously, we called directly into IMMS#{show,hide}SoftInput, but with the refactor, we only set the IME visibility. Therefore, adjusting the test.

Test: atest FrameworksInputMethodSystemServerTests:com.android.server.inputmethod.InputMethodManagerServiceWindowGainedFocusTest
Flag: android.view.inputmethod.refactor_insets_controller
Fix: 355145573
Change-Id: I70c8a1d625304182f840ede792a9b9a8dc7e9234