Allow IME to register system back callbacks in

ImeOnBackInvokedDispatcher#switchRootView.

This has the same purpose as ag/20766528, which only addressed one of
the two occurances of IME registering callbacks.

Bug: 262727745
Test: CtsInputMethodTestCases
Test: KeyboardVisibilityControlTest

Change-Id: I0ad892f17211a132d8a69b7f68a6c10f5740c457