[Bug fix] make engagementMedium not required to fix issues in android sdk

[Sam Beveridge (00salmon)]

Description of the change

engagementMedium isn't required in the backend as it defaults to UNKNOWN, so the frontend query shouldn't require it either.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation or Development tools (readme, specs, tests, code formatting)

Links

• Jira issue number: (PUT IT HERE)
• Process.st launch checklist: (PUT IT HERE)

Checklists

Development

• Prettier was run (if applicable)
• The behaviour changes in the pull request are covered by specs
• All tests related to the changed code pass in development

Paperwork

• This pull request has a descriptive title and information useful to a reviewer
• This pull request has a Jira number
• This pull request has a Process.st launch checklist

Code review

• Changes have been reviewed by at least one other engineer
• Security impacts of this change have been considered