Block: Assign block list ref from block child

[aduth]

Regression introduced in #3171

This pull request is a partial revert of #3171, seeking to address an error which occurs when removing a block. The changes in #3171 depend to access props of a rendered element ref are insufficient, since the ref callback is invoked with null when the element unmounts. The changes here partially restore the original behavior prior to #3171, with an optimization to still use a constant reference to binding the block ref in VisualEditorBlockList by passing UID from the child Block component's own ref callback.

Testing instructions:

Verify that no errors occur when removing a block.

[youknowriad]

Interestingly, this bug has been catched by the e2e tests when I rebased #3069

[codecov]

Codecov Report

Merging #3205 into master will decrease coverage by 0.34%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #3205      +/-   ##
=========================================
- Coverage   31.54%   31.2%   -0.35%     
=========================================
  Files         221     221              
  Lines        6311    6381      +70     
  Branches     1121    1148      +27     
=========================================
  Hits         1991    1991              
- Misses       3632    3675      +43     
- Partials      688     715      +27

Impacted Files	Coverage Δ
editor/modes/visual-editor/block.js	0% <0%> (ø)	⬆️
editor/modes/visual-editor/block-list.js	0% <0%> (ø)	⬆️

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