Update minSdkVersion to 19 in AndroidManifest and tests

[gmackall]

Flutter is deprecating support for android api levels 16, 17, and 18 (they have already been removed from https://docs.flutter.dev/reference/supported-platforms). See discussion on flutter/flutter#82000.

This PR updates minSdkVersions in the AndroidManifest and in the tests, where it was less than 19.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
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• I updated/added relevant documentation (doc comments with ///).
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• All existing and new tests are passing.

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[gmackall]

Converting back to draft until I get around to fixing these test failures (of course the final two tests fail as soon as I mark ready for review 🤦 )

[flutter-dashboard]

This pull request has been changed to a draft. The currently pending flutter-gold status will not be able to resolve until a new commit is pushed or the change is marked ready for review again.

[gmackall]

Closing until I have more time to actively work on this