fix(module:avatar): fix avatar text resize when used in list

[wzhudev]

close #2005

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?

[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: #2005

What is the new behavior?

Avatar text would resize correctly.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

When avatar is rendered in list, its DOM is not rendered before calcStringSize gets invoked. Need to write a test case for this?

[codecov]

Codecov Report

Merging #2074 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2074      +/-   ##
==========================================
+ Coverage   95.96%   95.96%   +<.01%     
==========================================
  Files         473      473              
  Lines       11516    11519       +3     
  Branches     1535     1535              
==========================================
+ Hits        11051    11054       +3     
  Misses        132      132              
  Partials      333      333

Impacted Files	Coverage Δ
components/avatar/nz-avatar.component.ts	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cca1fdd...0e97472. Read the comment docs.

[vthinkxie]

try https://material.angular.io/cdk/observers/overview
ref https://developer.mozilla.org/zh-CN/docs/Web/API/MutationObserver

[hsuanxyz]

close via #2536