Update RelativeDateFormat.java

[xuxiawei]

simplify logic and improve performance

What's the purpose of this PR

XXXXX

Which issue(s) this PR fixes:

Fixes #

Brief changelog

XXXXX

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[github-actions]

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[xuxiawei]

I have read the CLA Document and I hereby sign the CLA

[codecov-commenter]

Codecov Report

Merging #4111 (da83dee) into master (046a13d) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #4111      +/-   ##
============================================
- Coverage     52.49%   52.43%   -0.06%     
+ Complexity     2614     2611       -3     
============================================
  Files           484      484              
  Lines         15206    15206              
  Branches       1572     1572              
============================================
- Hits           7983     7974       -9     
- Misses         6671     6680       +9     
  Partials        552      552              

Impacted Files	Coverage Δ
...amework/apollo/portal/util/RelativeDateFormat.java	0.00% <0.00%> (ø)
...work/apollo/biz/message/DatabaseMessageSender.java	50.00% <0.00%> (-14.59%)	⬇️
.../apollo/internals/RemoteConfigLongPollService.java	77.10% <0.00%> (-1.21%)	⬇️

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[nobodyiam]

LGTM