Simplify ImeTargetChangeListener impl a bit

This is a mechanical refactoring CL before making

  ImeVisibilityStateComputer

multi-user aware.

There must be no observable behavior change.

Bug: 356177006
Test: presubmit
Flag: EXEMPT refactor
Change-Id: Ib8843357affc225ce1c224e4530b5c54341d7007