[cloud_functions] Replace Kotlin with Java

[sbeitzel]

Description

The no-op Android plugin for package:cloud_functions_web was the default skeleton created by flutter create -t plugin . which, as it turns out, doesn't actually compile in the CI environment. This changes the implementation from Kotlin to Java and removes all references to Kotlin from the Android plugin.

Related Issues

• the issue surfaced during checks for PR 1909
• This is all in service of [cloud_functions] Add web support flutter/flutter#45299

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I signed the [CLA].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• No, this is not a breaking change.