Refactor `avatar` to make sure it never fails to generate a valid hex.

[Elijbet]

Description

private generateFillColor function should never generate an invalid hex

Proposed Advantages

Simplify code to prevent errors in coordinating background and foreground colors, as this is important for validating contrast.

Which Component

avatar

Relevant Info

Stems from https://github.com/Esri/calcite-components/pull/6528 - refer to this comment for specifics in the PR.