Fix IME callback being unexpectedly garbage-collected and breaking back

gesture.

Bug: 266528689
Test: Open GMail, Calendar, Launcher, find UI surfaces to bring up IME,
then swipe back. Repeat in different combinations of the steps and make
sure back always works.
Test: atest WindowOnBackInvokedDispatcherTest

Change-Id: I0cba6d6963c6b988f02a7baf56f2901d17b5c9b3