Clean up InputMethodBindingController a bit

This CL does several minor clean-ups for InputMethodBindingController.

 - Removed an unused field mRes.
 - Added a missing @GuardedBy("ImfLock.class") annotation.
 - Inlined bindCurrentInputMethodServiceVisibleConnection().

There must be no observable behavior change.

Bug: 234882948
Test: presubmit
Change-Id: Idc7371dd15f53c39ea508633bce9a30e56c773cb