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

[HyperLife1119]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• 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?

• 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
• No

Other information

[zorro-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.15%. Comparing base (c64e1a1) to head (aba6f8e).
Report is 86 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8754      +/-   ##
==========================================
+ Coverage   91.67%   93.15%   +1.47%     
==========================================
  Files         538      538              
  Lines       18457    18455       -2     
  Branches     2816     2816              
==========================================
+ Hits        16920    17191     +271     
- Misses       1225     1226       +1     
+ Partials      312       38     -274     

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[Laffery]

LGTM