Use @EnforcePermission for IIMM#showInputMethodPickerFromSystem()

This CL manually migrates

  IInputMethodManager#showInputMethodPickerFromSystem(boolean, int)

from manual permission check with

  Context#checkCallingPermission(WRITE_SECURE_SETTINGS)

to code-generation check with

  @EnforcePermission("WRITE_SECURE_SETTINGS").

There should be no observable semantic behavior change in this CL.

Bug: 34886274
Bug: 232058525
Bug: 237316307
Test: manually verified that SecurityException is thrown
Change-Id: I9d1be1aa65276fcc50c2868fed6b0447d54ff7ee

core/java/com/android/internal/view/IInputMethodManager.aidl

@@ -76,6 +76,7 @@ interface IInputMethodManager {
     void showInputMethodPickerFromClient(in IInputMethodClient client,
             int auxiliarySubtypeMode);
 
+    @EnforcePermission("WRITE_SECURE_SETTINGS")
     @JavaPassthrough(annotation="@android.annotation.RequiresPermission(value = "
             + "android.Manifest.permission.WRITE_SECURE_SETTINGS)")
     void showInputMethodPickerFromSystem(in IInputMethodClient client,

services/core/java/com/android/server/inputmethod/InputMethodManagerService.java

@@ -3993,15 +3993,10 @@ public final class InputMethodManagerService extends IInputMethodManager.Stub
         }
     }
 
+    @EnforcePermission(Manifest.permission.WRITE_SECURE_SETTINGS)
     @Override
     public void showInputMethodPickerFromSystem(IInputMethodClient client, int auxiliarySubtypeMode,
             int displayId) {
-        if (mContext.checkCallingPermission(android.Manifest.permission.WRITE_SECURE_SETTINGS)
-                != PackageManager.PERMISSION_GRANTED) {
-            throw new SecurityException(
-                    "showInputMethodPickerFromSystem requires WRITE_SECURE_SETTINGS "
-                            + "permission");
-        }
         // Always call subtype picker, because subtype picker is a superset of input method
         // picker.
         mHandler.obtainMessage(MSG_SHOW_IM_SUBTYPE_PICKER, auxiliarySubtypeMode, displayId)