content: Open links in-app on Android

[rajveermalviya]

url_launcher plugin now supports the desired behavior, which is using Android Custom Tabs, so we don't need the workaround of opening the links in external browser anymore, thus removed them.

Upstream PR:
flutter/packages#4739

Fixes #279

[gnprice]

Thanks @rajveermalviya! This looks great — merging, with small changes:

• I pulled out the change to our dependencies as its own prep commit. If you look at our past dependency-updating commits, you'll see we keep them separate from code changes; try either the gitk graphical Git viewer, like gitk pubspec.*, or a command like git log --stat -p --full-diff pubspec.* plus my "secret" for using git log -p .
  • I also narrowed down that commit to make just the one change related to the rest of this PR.
• I updated some tests that appeared when rebasing this onto main.

It'll be great to have this fix. Sorry I've been distracted and took so long to come back and integrate it.