fix(datepicker): access view child from ngAfterViewInit (#3152)
fix(datepicker): access view child from ngAfterViewInit (#3152)

Datepicker accessed a view child from the `ngAfterContentInit` hook instead of accessing it from the `ngAfterViewInit` hook.

This doesn't work anymore with Ivy. That is a regression in Ivy, but is also an issue with ng-bootstrap which doesn't respect the standard life-cycle.
Besides, the component didn't implement the `AfterContentInit` interface.
It now implements the `AfterViewInit` interface.

Fixes #3150
benouat/ng-bootstrap,fbasso/ng-bootstrap,ng-bootstrap/ng-bootstrap,maxokorokov/core,fbasso/ng-bootstrap,maxokorokov/core,fbasso/ng-bootstrap,benouat/ng-bootstrap,benouat/ng-bootstrap,jnizet/ng-bootstrap,ng-bootstrap/ng-bootstrap,benouat/ng-bootstrap,jnizet/ng-bootstrap,maxokorokov/core,ng-bootstrap/ng-bootstrap,jnizet/ng-bootstrap,fbasso/ng-bootstrap,jnizet/ng-bootstrap,ng-bootstrap/ng-bootstrap,maxokorokov/core