Update receiving dispatcher to ImeOnBackDispatcher after IMM gain focus

The InputMethodManager can lost mCurRootView after lost focus, such as
screen off. So when screen turned on and IME reshow, the
ImeOnBackDispatcher may not able to set OnBackInvokedCallback to
mCurRootView immediately, there should wait for IMM re-gain focus.

Flag: EXEMPT bugfix
Bug: 355006526
Test: atest KeyboardVisibilityControlTest
Change-Id: I947f113d7c7b0e79b41d2ac848407a56db9f1fef
2 files changed