[firebase_dynamic_links] Plugin should not crash if activity is unavailable

[mehmetf]

See: flutter/plugins#1989

Currently, if registrar.activity() is not there, the plugin crashes. This is a legitimate use case for some apps since they invoke the Dart isolate from application class (as opposed to activity). We should avoid using activity if we can (e.g. we don't need it for Task objects).

[mehmetf]

Fixed with #14