Inline IMMS#hasAttachedClient() with making it multi-user aware

This CL inlines

  InputMethodManagerService#hasAttachedClient()

with also propagating the correct user ID rather than always using

  InputMethodManagerService#mCurrentUserId.

The observable behavior should remain to be the same for single user
mode, and for concurrent multi-user mode the new behavior should be
the right one.

Bug: 350386877
Test: presubmit
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: I9befe37eac3d40d0140f063305a9b92c3bf1f8dc