Handle ImeTracker.Token missing flows

Created an empty ImeTracker.Token instance for testing,
and added more reasons to handle IME flows previously
missed from tracking.

Test: atest InputMethodStatsTest
  ImeInsetsSourceConsumerTest
  InputMethodManagerServiceWindowGainedFocusTest
  DefaultImeVisibilityApplierTest
  ImeVisibilityStateComputerTest#testOnApplyImeVisibilityFromComputer
Bug: 271426908
Change-Id: Idfa8dabf16d0bcd43ff0c0d64affb2e5f923f44b