fix(popover): get back default 15px offset

[nnixaa]

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:

[codecov]

Codecov Report

Merging #1201 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1201   +/-   ##
=======================================
  Coverage   79.53%   79.53%           
=======================================
  Files         202      202           
  Lines        6246     6246           
  Branches      495      495           
=======================================
  Hits         4968     4968           
  Misses       1162     1162           
  Partials      116      116

Impacted Files	Coverage Δ
...work/theme/components/popover/popover.directive.ts	91.17% <ø> (ø)	⬆️
...nts/cdk/overlay/dynamic/dynamic-overlay-handler.ts	100% <100%> (ø)	⬆️