Fix typo "SetTestFluttterStub", "FluttterActivity" and "implementantions"

[nilsreichardt]

As recommended in #33547 (comment), I fixed all typos in one PR.

Fixes flutter/flutter#104366
Fixes flutter/flutter#104367
Fixes flutter/flutter#104368
Fixes flutter/flutter#104369
Fixes flutter/flutter#104370

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

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[chinmaygarde]

LGTM on the typo fixes.

In a separate patch, we should figure out how to dry up the stubs. cc @cbracken

[nilsreichardt]

Do I need to request a test exempt?

[cbracken]

Do I need to request a test exempt?

@nilsreichardt yep. I've just pinged @Hixie on Discord for a test exemption.

[nilsreichardt]

@cbracken Great, thanks!

[Hixie]

test-exempt: only affects comments