fix(ui5-avatar): Correct colors are applied for acc themes

[vladitasev]

For normal themes, ui5-avatar fulfilled the Fiori design, but for accessibility themes, there are different requirements:

• there must be a border for icon and initial type avatars
• background and text color must be purely black/white (no accent colors).

This change extracts all necessary variables to fulfill this.

Expected design:

closes: #1780