fix(module: pull-to-refresh): fix pull-to-refresh not work bug & code refactor

[BronzeCui]

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-mobile/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: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #573 into master will increase coverage by 0.05%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #573      +/-   ##
==========================================
+ Coverage   88.52%   88.58%   +0.05%     
==========================================
  Files         270      270              
  Lines        7601     7593       -8     
  Branches     1084     1082       -2     
==========================================
- Hits         6729     6726       -3     
+ Misses        428      425       -3     
+ Partials      444      442       -2

Impacted Files	Coverage Δ
...nents/pull-to-refresh/pull-to-refresh.component.ts	73.88% <66.66%> (+1.76%)	⬆️

---

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 5aa3149...c52bca9. Read the comment docs.