fix(explore): DndColumnSelect sometimes not working with multi: false

[kgabryje]

SUMMARY

Due to incorrect handling of multi: false prop in DndColumnSelect, we sometimes passed an array of arrays of column names, when the chart was expecting an array of column names. Moreover, I noticed that sometimes when we create a chart and then change dataset, chart still tries to use some column names from the old dataset.
This PR fixes both issues.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before: see #15532
After:
https://user-images.githubusercontent.com/15073128/125910050-317b3f22-8ccd-4a42-b14d-06c01cb38421.mov

TESTING INSTRUCTIONS

0. Set ENABLE_EXPLORE_DRAG_AND_DROP to True
1. Create a chart that uses controls that accept only single column, e.g. Chord
2. Verify that chart is created correctly
3. Change dataset and try to create a chart again
4. Verify that there are no "leftovers" from previous dataset (before, sometimes chart tried to use column names from previous dataset even though they were removed from controls)

ADDITIONAL INFORMATION

• Has associated issue: fixes [Explore-drag&drop ]DnD control returns an array of columns when it’s a single selection #15532
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

CC: @junlincc

[codecov]

Codecov Report

Merging #15731 (6160881) into master (0721f54) will increase coverage by 0.00%.
The diff coverage is 60.71%.

❗ Current head 6160881 differs from pull request most recent head 48d9596. Consider uploading reports for the commit 48d9596 to get more accurate results

@@           Coverage Diff           @@
##           master   #15731   +/-   ##
=======================================
  Coverage   76.91%   76.92%           
=======================================
  Files         983      983           
  Lines       51602    51609    +7     
  Branches     6980     6988    +8     
=======================================
+ Hits        39688    39698   +10     
+ Misses      11691    11688    -3     
  Partials      223      223           

Flag	Coverage Δ
javascript	71.36% <60.71%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
...ontrols/DndColumnSelectControl/DndColumnSelect.tsx	64.44% <59.09%> (+4.44%)	⬆️
...ols/DndColumnSelectControl/utils/optionSelector.ts	46.15% <66.66%> (+11.67%)	⬆️

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[zhaoyongjie]

LGTM! thanks for the fix.

[geido]

/testenv up FEATURE_ENABLE_EXPLORE_DRAG_AND_DROP=true

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