Don't call sharedApplication in App Extension

[ccorcos]

Related to this bug: #25769

Note I also had to add to the bottom of my podfile because RCTLinking had APPLICATION_EXTENSION_API_ONLY='YES' by default somehow.

post_install do |installer|
  installer.pods_project.targets.each do |target|
    target.build_configurations.each do |config|
      config.build_settings['APPLICATION_EXTENSION_API_ONLY'] = 'NO'
    end
  end
end

[pull-bot]

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[zhongwuzw]

Thanks! We need to replace all UIApplication.sharedApplication with RCTSharedApplication().

[facebook-github-bot]

@sammy-SC has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @ccorcos in 81733d9.

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