Add missing built-in user confirmation functionality to modSchema()

This change implements the functionality built directly into libanki which allows a user to abort an operation which will require a full sync. It works a bit different from the Python since we're not allowed to show blocking dialogs in Android.

This is currently inconsistent with the original python in a couple of places, but @dae is going to fix those inconsistencies upstream over the next few days, so let's wait for his commits to be finalized before merging these changes.