checkbox: sizing does not match spec or accessibility guidance

[aipsensmall]

Actual Behavior:

• What is the issue? * Checkboxes (selected and not) are 20px squares. They are also not within a 24px bounding box.
• What is the expected behavior? As per Material icon library, checkboxes should be 18px squares within a 24px by 24px square container.

@ErinCoughlan

[clshortfuse]

This PR has been on the backburner, but it would close this issue: #8713