fix(all): Add additional consumer proguard rules for missing classes

[mattcreaser]

• PR title and description conform to Pull Request guidelines.

Issue #, if available: #2998

Description of changes:
Add consumer rules to skip warnings introduced in 2.26.0. The OkHttp4Engine is only referenced safely by Amplify so this is OK, and the Immutable reference is for a static linting annotation and can safely be ignored.

How did you test these changes?
Verified r8 no longer produces warnings for these missing classes.

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.