fix(module:avatar): calculate size at the right time

[HyperLife1119]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• [x] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Documentation content changes
• [ ] Application (the showcase website) / infrastructure changes
• [ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

当 文字类型的头像 在像 dropdown 这种 overlay 组件中渲染时，无法正确计算文本尺寸：

What is the new behavior?

通过使用 afterRender 来确保可以正确 读/写 DOM 来解决该问题：

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information