Simplify IMMS#shouldShowImeSwitcherLocked()

This is a mechcanical refactoring CL that aims to simplify the code by
taking advantage of per-user InputMethodBindingController instance we
recently introduced [1].

There must be no observable behavior change.

 [1]: Id6e9eca6caaa8edde8c4a405448c31c0ca986509
      2238ecd4633b60b87f8bf34fc027a8780082d235

Bug: 325515685
Test: presubmit
Change-Id: I40cf0ce1260df703d542fff66bf3511cf8fa1f59
diff --git a/services/core/java/com/android/server/inputmethod/InputMethodManagerService.java b/services/core/java/com/android/server/inputmethod/InputMethodManagerService.java
index a43d6af..189e845 100644
--- a/services/core/java/com/android/server/inputmethod/InputMethodManagerService.java
+++ b/services/core/java/com/android/server/inputmethod/InputMethodManagerService.java
@@ -2635,7 +2635,9 @@
         if (mMenuController.getSwitchingDialogLocked() != null) return false;
         // When we are switching IMEs, the IME switcher button should be hidden.
         final var userData = mUserDataRepository.getOrCreate(mCurrentUserId);
-        if (!Objects.equals(userData.mBindingController.getCurId(), getSelectedMethodIdLocked())) {
+        final var bindingController = userData.mBindingController;
+        if (!Objects.equals(bindingController.getCurId(),
+                bindingController.getSelectedMethodId())) {
             return false;
         }
         if (mWindowManagerInternal.isKeyguardShowingAndNotOccluded()