Do not forward & register IME compat back callback in the app process.

IME window's ViewRootImpl registers its compat callback when the first child view is added. If the app dispatches start input and sets a forwarding dispatcher on the IME's WindowOnBackInvokedDispatcher, before IME ViewRootImpl receives its first child, the compat callback would be forwarded to app process and registered on the app window.

We should prevent this from happening because the IME window never has focus and the compat callback will just get stuck and break back nav.

Test: m -j.
Test: for run in {1..500};  do adb shell input tap 164 1950; sleep 1; adb shell input swipe 50 1000 200 1000; sleep 1; done
Bug: 269821450
Change-Id: I9157d8d9c85edc29a5d004709c2cd3e25764b56a