Import EnforcePermission in IMMS

This is a follow up CL for a recent global clean-up CL [1], which
migrated

  IInputMethodManager#removeImeSurface()

from manual permission check with

  Context.enforceCallingPermission(INTERNAL_SYSTEM_WINDOW)

with

  @EnforcePermission("INTERNAL_SYSTEM_WINDOW").

To minimize the risk of merge conflicts, that CL did not import
EnforcePermission.  Now that the CL gets merged successfully, let's
simplify the code for future use of @EnforcePermission attribute in
InputMethodManagerService.

This is a mechanical refactoring.  There should be no behavior change.

 [1]: Id9bdeb71e84962d55fd1657e91856d84e1c3d11b
      f48d794e

Bug: 234882948
Test: presubmit
Change-Id: Id7d02ef0055a65c1e4527afff43959444418f751