Remove notifyImeHidden call in ImeBackAnimationController

Calling notifyImeHidden had two purposes:
 1. Triggering the regular IME hide animation. This can be replaced with
    InsetsController#hide where applicable.
 2. Triggering the back callback unregistration by the
    InputMethodService. This is not required anymore since we are using
    ImeOnBackInvokedDispatcher#preliminaryClear to do so instead.

Bug: 417738140
Test: ImeBackAnimationControllerTest
Test: Manual, i.e. verified IME back navigation in multiple setups:
      predictive back enabled vs disabled, splitscreen, folded and
      unfolded screen, quick double back swipes, quick input field
      refocus
Flag: EXEMPT bugfix
Change-Id: I01278c31b5a98e66d0eee7aea6927b79828e06f4