[firebase_crashlytics] make sure the keys are actually added to the returned map

[spockfish]

Description

In the 'prepareKeys()' method the keys needed to be added to a map that is being returned. In the method there's an iteration over the keys, but the keys are not added to the resulting map. This leads to the issue that custom keys are not being reported to Crashlytics. This PR fixes that.

Related Issues

Fixes flutter/flutter#32873

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Breaking Change

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[collinjackson]

Thanks so much for your contribution. I've added tests to your PR. In the future please try to include tests -- it really saves us a lot of time reviewing.