Open links in-app by default on Android, once that has a good UX

[gnprice]

When opening an external link, we have a choice whether to do so within the app or in an external browser.

Currently we do so following the upstream Flutter defaults (documented at launchUrl), except that on Android we always use an external app rather than the in-app browser. That's because Flutter's implementation of the latter is currently not a good UX:

• Use Chrome Custom Tab in url_launcher, instead of custom WebView activity flutter/flutter#18589 (comment)

Once that's fixed, I think it'd be best to default on Android to opening in-app, just as the Flutter url_launcher plugin defaults to. That's what we chose on zulip-mobile, for reasons discussed in this chat thread, and I think all the same reasoning still applies.

Separately:

• We should offer an option (again just as we do in zulip-mobile), so that this is only about the default. That's part of Store some client settings #97.
• On iOS our choice in zulip-mobile was to default to an external app, unlike on Android. That reasoning is in the same thread and again I think it all still applies. That differs from the url_launcher default, so we should adjust that: Open links externally by default on iOS #280

[gnprice]

Milestoning as post-launch based on the scenario where it falls to us to fix the upstream issue. But if someone else picks up that upstream issue and fixes it sooner, then what's left of this issue will be an easy tweak and we should go ahead and do it.

[rajveermalviya]

I opened flutter/packages#4739 which uses Android Custom Tabs when launchUrl is called with LaunchMode.externalApplication. So, after that is maybe merged we can change to use that for android platform.