[share] Do not tear down channel onDetachedFromActivity.

[zhenqiu1101]

We should not tear down the method channel onDetachedFromActivity. That will cause MissingPluginException for the following method calls. internal: b/183065251. Open source issue: flutter/flutter#12264

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[zhenqiu1101]

Open source issue: flutter/flutter#12264

[xster]

This is sooooo good! Awesome find! This totally makes sense. The current implementation doesn't work since it's not symmetrical between engine attach/detach and activity attach/detach.

[zhenqiu1101]

@cyanglaz

Hi Chris, could you advice on the test failures?

[zhenqiu1101]

Chris is on leave. @stuartmorgan could you please advice on the test failures? It looks unrelated the PR. iOS share test and url_launcher plugin. Wanna make sure that the PR is safe to merge. Thanks!

[stuartmorgan]

We can land this since it's affecting an internal customer, but we're in the process of turning this plugin down in favor of https://pub.dev/packages/share_plus; see the notice in CONTRIBUTING.md. You should submit this fix there as well since at some point there should be a transition to that version, and you'll want to have this fix there as well.