Unregister IME callback in onBackInvoked immediately

By removing the IME callback in onBackInvoked immediately, a new back gesture that starts during the IME hide animation can already dispatch to a new back callback. Currently, back gestures that start during the (predictive back) IME hide animation are simply ignored. This CL fixes that.

Bug: 322836622
Flag: ACONFIG android.view.inputmethod.predictive_back_ime DISABLED
Test: separate CL
Change-Id: Iab628e23982915b5c5f17896436157794e90b649