Deflake CloseImeOnDismissPopupDialogTest

In imeLayerAndImeSnapshotVisibleOnScreen, add "isOptional = true" when
verifying IME snapshot isVisible, in case a false alarm flakiness
happens when the IME snapshot surface removed when the popup editor
dialog was dismissed.

Bug: 274653385
Test: atest FlickerTests:CloseImeOnDismissPopupDialogTest
--rerun-until-falure 10 passed on cuttlefish devices.

Change-Id: I61bf11a5b63b5a540e3693f33253b2a599c2142d