[firebase_auth]Fixed linkWithCredential implementation for Android

[surengrig]

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[cqhchan]

Hi, I have made some fixes for this plugin at #973 to include cases on android phone when verification is handled internally

[surengrig]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

[collinjackson]

Thanks for the contribution. It looks like this was fixed in #1333