Do not register OnBackInvokedCallback for SoftInputWindow.

By default Dialog#onStart will register onBackInvokedCallback, which
should be necessary since InputMethodService will register
CompatOnBackInvokedCallback for input method window.

Bug: 294162707
Test: atest BackGestureInvokedTest
Test: manual, launch Calendar then trigger back several times, verify
the back gesture can close app normally.
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:da6336c35396bbea2191cb0e6b4aa72272a8059a)
Merged-In: Icacfef60666fba1522f9d0f4ddc47e74a4560a32
Change-Id: Icacfef60666fba1522f9d0f4ddc47e74a4560a32