Move #mLastImeTargetWindow to ImeVisibilityStateComputer

This is a mechanical refactoring to move

  InputMethodManagerService#mLastImeTargetWindow

to ImeVisibilityStateComputer, which will soon become per-user
instance.

There must be no observable behavior change.

Bug: 349904272
Test: presubmit
Flag: EXEMPT refactor
Change-Id: Ib47886800cd7efb4ba5e5055113b9c1e6404e2b8
3 files changed