Remove mImeInsetsFrozenUntilStartInput

Since shell transition enabled, postApplyAnimation(visible=false)
is only called when the activity is completed invisible, rather
than transition ready in legacy transition. So the flag no longer
affects the appearance on screen.

Also the invisible-request state should already have the effect
as frozen insets by setInsetsFrozen/isReadyToDispatchInsetsState.

Bug: 365884835
Flag: EXEMPT remove outdated code
Test: atest WindowStateTests
Test: Open IME on Play and Messages, use gesture navigation
      to switch the tasks. The IME on the apps won't disappear
      during transition.
Change-Id: I697e4c93e2bd12971fb2104ccfa8170268a380b0