Bug 1870232 - Delegate the Activity's getNavDirections() functionality

[t-p-white]

Step 2 from Bug 1865306 - RFC Custom Tabs

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https://bugzilla.mozilla.org/show_bug.cgi?id=1870232

[MozillaNoah]

Now that we're in a new release cycle, I'm cool with approving this 👍

[t-p-white]

Now that we're in a new release cycle, I'm cool with approving this 👍

Thanks @MozillaNoah! While resolving the conflict I've also moved the code from the separate 'navigation' file to be in the Activity extension file based on feedback on another review

[t-p-white]

Latest push updates the nav_graph to set the default arg to null so we no longer need to provide this explicitly 🙂

cc @MozillaNoah