Currently the 'permissionCallback' object is only available at the Verto
instance level. This is problematic in multi-call scenarios, where an
individual call dialog needs access to the onGranted/onDenied callback
functions.

The patch adds a check for existence of onGranted/onDenied callback functions
at the dialog level, with a fallback to the original behavior of calling
onGranted/onDenied from the Verto instance if it's not available on the dialog.
This preserves backwards compatibility while allowing per-dialog overrides
going forward.