Inline IMMS#getCurTokenDisplayIdLocked()

This CL mechanically inlines

  InputMethodManagerService#getCurTokenDisplayIdLocked().

The observable behavior should remain to be the same for single user
mode, and for concurrent multi-user mode it remains to be broken until
we make

  InputMethodManagerService#mImeDrawsImeNavBarRes

multi-user aware anyway.

Bug: 345519864
Bug: 350386877
Test: presubmit
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: Id0136efc7e6edab057c3adaa1e8603448c89816a