fix(accordion): extract inner elements styles outside accordion

[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:

This way nb-accordion inner items (like nb-accordion-item-header, nb-accordion-item, nb-accordion-item-body) keep their styles when are placed outside nb-accordion element (for example, when used as cdkDrag as when element dragged it placed in the body).

[codecov]

Codecov Report

Merging #2536 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2536   +/-   ##
=======================================
  Coverage   78.93%   78.93%           
=======================================
  Files         253      253           
  Lines        7808     7808           
  Branches      855      855           
=======================================
  Hits         6163     6163           
  Misses       1380     1380           
  Partials      265      265           

Impacted Files	Coverage Δ
...k/theme/components/datepicker/datepicker.module.ts	100.00% <ø> (ø)
...heme/components/timepicker/timepicker.directive.ts	22.95% <ø> (ø)
...k/theme/components/timepicker/timepicker.module.ts	50.00% <ø> (ø)