feat(module:affix anchor backtop): refactor affix anchor backtop

[cipchk]

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
[x] 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

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[coveralls]

Coverage decreased (-0.1%) to 98.076% when pulling bb2e3f5 on cipchk:affix into abb7de0 on NG-ZORRO:0.7.0.

[codecov]

Codecov Report

Merging #1025 into 0.7.0 will increase coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##            0.7.0    #1025      +/-   ##
==========================================
+ Coverage   97.88%   98.01%   +0.13%     
==========================================
  Files         149      156       +7     
  Lines        5159     5448     +289     
  Branches      620      656      +36     
==========================================
+ Hits         5050     5340     +290     
+ Partials      109      108       -1

Impacted Files	Coverage Δ
components/anchor/nz-anchor.component.ts	100% <100%> (ø)
components/back-top/nz-back-top.module.ts	100% <100%> (ø)
components/affix/nz-affix.component.ts	100% <100%> (ø)
components/back-top/nz-back-top.component.ts	100% <100%> (ø)
components/anchor/nz-anchor-link.component.ts	100% <100%> (ø)
components/anchor/nz-anchor.module.ts	100% <100%> (ø)
... and 4 more

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