Fix issue where max number of multi-entity detector doesn't work for UpdateDetector

[yizheliu-amazon]

Issue #, if available:

Description of changes:
Fix issue where max number of multi-entity detector doesn't work for UpdateDetector

Currently we only check if max number of multi-entity detector is reached for CreateDetector, this is to add that check for UpdateDetector.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Merging #285 into master will increase coverage by 0.02%.
The diff coverage is 71.42%.

@@             Coverage Diff              @@
##             master     #285      +/-   ##
============================================
+ Coverage     70.03%   70.06%   +0.02%     
- Complexity     1815     1821       +6     
============================================
  Files           190      190              
  Lines          8867     8879      +12     
  Branches        756      757       +1     
============================================
+ Hits           6210     6221      +11     
- Misses         2301     2302       +1     
  Partials        356      356              

Flag	Coverage Δ	Complexity Δ
#cli	79.27% <ø> (ø)	0.00 <ø> (ø)
#plugin	69.25% <71.42%> (+0.03%)	1821.00 <4.00> (+6.00)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...est/handler/IndexAnomalyDetectorActionHandler.java	48.77% <71.42%> (+0.06%)	32.00 <4.00> (+4.00)
...sticsearch/ad/indices/AnomalyDetectionIndices.java	50.33% <0.00%> (-0.68%)	40.00% <0.00%> (-1.00%)
...stroforelasticsearch/ad/util/RestHandlerUtils.java	86.84% <0.00%> (+2.63%)	14.00% <0.00%> (+1.00%)
...asticsearch/ad/cluster/ADClusterEventListener.java	92.00% <0.00%> (+4.00%)	14.00% <0.00%> (+1.00%)
...ransport/DeleteAnomalyDetectorTransportAction.java	57.95% <0.00%> (+4.54%)	16.00% <0.00%> (+1.00%)