fix: (module: tabbar): fix tabbar set activeTab not work and add demo

[Guoyuanqiang]

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?

[ ] 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
[ ] No

Other information

[codecov]

Codecov Report

Merging #345 into master will decrease coverage by 0.07%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #345      +/-   ##
==========================================
- Coverage   89.64%   89.56%   -0.08%     
==========================================
  Files         229      229              
  Lines        6873     6878       +5     
  Branches      561      562       +1     
==========================================
- Hits         6161     6160       -1     
- Misses        548      553       +5     
- Partials      164      165       +1

Impacted Files	Coverage Δ
components/tab-bar/tab-bar.component.ts	84.31% <66.66%> (-2.65%)	⬇️
components/picker/picker.service.ts	88.88% <0%> (-5.56%)	⬇️
components/picker/picker.component.ts	80.58% <0%> (-1.77%)	⬇️

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