Prevent "new mode" and "choose schedule" dialogs from being created without a listener

This can happen if the activity is recreated; our approach doesn't support this case very well. With this change, the dialog is not reshown when the activity is recreated, which is not optimal but better than crashing (as it does today). There is no lost work because it's the first choice, and the follow-on steps (icon picker, etc) can be recreated without problems.

Fixes: 359746551
Test: manual, with don't keep activities
Flag: android.app.modes_ui
Change-Id: I84bdeb0007e8c50ec9dd8af61991c7e55ddb8298