Implement external view embedder on Android

[blasten]

Description

This PR implements the external view embedder (C++ side). There's a bit of refactoring that was required to make the code unit testable. The refactor primarily consisted on making public interfaces that can be compiled with the host toolchain.

I saw that gmock can be used on virtual methods to assert method calls such as the calls to Java.

Related Issues

flutter/flutter#55270

Tests

I added the following tests: Unit tests.

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.
• I signed the CLA.
• I read and followed the C++, Objective-C, Java style guides for the engine.
• I read the tree hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation.
• All existing and new tests are passing.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.
  • I wrote a design doc: https://flutter.dev/go/template Replace this with a link to your design doc's short link
  • I got input from the developer relations team, specifically from: Replace with the names of who gave advice
  • I wrote a migration guide: https://flutter.dev/go/breaking-changes-template Replace this with a link to a pull request that adds the migration guide to https://flutter.dev/docs/release/breaking-changes

[blasten]

This can be moved to flow if the general pattern looks ok, so other unit tests can use the mock.

[blasten]

Brings gmock

[chinmaygarde]

Sounds good.