[firebase_crashlytics] Logging improvements

[dotdoom]

Description

Make logging more obvious and to the right point of verbosity.

This change also gets rid of XCode build warnings about potential security issue with format string.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
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[dotdoom]

Since there are now multiple concurrent PRs for firebase_crashlytics, updating pubspec.yaml and/or changelog will be a waste of time (due to version drift).

PTAL and comment whether this change LGTY and once it is, I will make changes to CHANGELOG and pubspec right away.

[kroikie]

LGTM, feel free to bump version

[dotdoom]

Done

[dotdoom]

@kroikie a friendly ping