fix(popover): add missing start and end positions to positions mapping

[yggg]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Closes #1147

[codecov]

Codecov Report

Merging #1904 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1904      +/-   ##
==========================================
+ Coverage   83.97%   83.98%   +0.01%     
==========================================
  Files         234      234              
  Lines        7674     7675       +1     
  Branches      672      672              
==========================================
+ Hits         6444     6446       +2     
+ Misses       1032     1031       -1     
  Partials      198      198

Impacted Files	Coverage Δ
...k/theme/components/cdk/overlay/overlay-position.ts	90.42% <100%> (+1.17%)	⬆️

[nnixaa]

@yggg, I believe we need a test here, don't we?