[firebase_dynamic_links] Fix Android crashing when a headless plugin register this plugin.

[madtocc]

If there's a headless plugin it won't be able to register this plugin (there will be no activity from the registrar).

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[bparrishMines]

Hi @madtocc,

Thanks for the contribution! I merged in master to this branch and will merge this once tests pass again.

[bparrishMines]

LGTM