FS-11281: Verto.newCall dialog callback overrides should be set before invite

FS-11281: Verto.newCall dialog callback overrides should be set before invite

Verto.newCall() permits passing custom callback functions per call that
override the default dialog callbacks inherited from the Verto object.
However, they are currently set after calling the invite() function, and it's
possible that some of these callbacks could be called during the invite()
function's execution.

To avoid a race condition, move setting these custom callbacks on the dialog
to happen before calling invite().