Make Alert not cancelable by default on Android

[JonnyBurger]

Summary

By default, an alert is cancelable on Android but not on iOS.
This PR changes the behavior so that the Alert is not dismissable on Android by default.

The motivation is that many developers develop on iOS and test on Android, and do forget to consider the case that the alert is dismissable.
Consistent behavior by default makes it easier to develop cross-platform apps in general.

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For context and for your consideration, I have started a discussion here with the topic of whether React Native should try to use OS defaults or be consistent between platforms:

react-native-community/discussions-and-proposals#121

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If this PR gets merged, the docs should be updated as well:

https://github.com/facebook/react-native-website/blob/master/docs/alert.md#android

Changelog

[Android] [Changed] - By default, alerts are not dismissable

Test Plan

By testing out the change in node_modules, I have found that the change has the intended effect.

[analysis-bot]

Code analysis results:

• eslint found some issues. Run yarn lint --fix to automatically fix problems.

[cpojer]

Thanks for this contribution! I appreciate that you are aligning this kind of UX stuff between platform, that totally makes sense.

[facebook-github-bot]

@cpojer is landing 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 @JonnyBurger in ec941cd.

^{When will my fix make it into a release? | Upcoming Releases}

[JonnyBurger]

Happy to do it! Thanks for taking the time to weigh in, review and merge.

I have also created a PR for docs to correctly reflect the new behavior: react/react-native-website#899