Fix/overlay components providers

[Tibing]

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:

Nebular Overlays now have to work properly without explicit importing NbOverlayModule in lazy loaded modules

Closes #854

[codecov]

Codecov Report

Merging #858 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #858      +/-   ##
==========================================
+ Coverage   76.94%   76.96%   +0.02%     
==========================================
  Files         201      201              
  Lines        5907     5913       +6     
  Branches      449      449              
==========================================
+ Hits         4545     4551       +6     
+ Misses       1260     1258       -2     
- Partials      102      104       +2

Impacted Files	Coverage Δ
...ramework/theme/components/dialog/dialog.service.ts	88.7% <100%> (+0.18%)	⬆️
...ramework/theme/components/toastr/toastr.service.ts	72.61% <100%> (+0.32%)	⬆️
.../theme/components/cdk/overlay/overlay-container.ts	93.1% <100%> (+1.1%)	⬆️
.../framework/theme/components/cdk/overlay/mapping.ts	89.88% <0%> (-1.13%)	⬇️
...framework/theme/components/toastr/toastr.module.ts	90.9% <0%> (+9.09%)	⬆️