[firebase_admob] update AGP, gradle and admob

[juliocbcotta]

Description

This PR

• updates AGP, gradle and admob
• Fix some warnings Android Studio was complaining

Warning example:

• variable.equals(constant) => constant.equals(variable) to avoid null pointer exception
• a reference for Activity was being kept global in a class where is was being used only locally, so it was removed.
• int a = call.argument(constant) => Integer a= call.argument(constant) to remove possible null pointer warning.

Related Issues

Checklist

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• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
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• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
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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).
• No, this is not a breaking change.

[collinjackson]

Can you include the warning removal here in the changelog?

[ened]

@collinjackson Let's get this merged?

[collinjackson]

Migrated to firebase/flutterfire#34