fix(module:slider): fix keydown not trigger nzOnAfterChange (#7252)

close #7251
* fix(module:slider): fix keydown not trigger nzOnAfterChange

* chore: clean up the code

* test(module:slider): add test case

* fix(module:slider): test case failed

* chore: remove unused code

components/slider/slider.component.ts

@@ -259,6 +259,7 @@ export class NzSliderComponent implements ControlValueAccessor, OnInit, OnChange
       ? (this.value as number[])[this.activeValueIndex!] + step
       : (this.value as number) + step;
     this.setActiveValue(ensureNumberInRange(newVal, this.nzMin, this.nzMax));
+    this.nzOnAfterChange.emit(this.getValue(true));
   }
 
   onHandleFocusIn(index: number): void {

components/slider/slider.spec.ts

@@ -918,6 +918,16 @@ describe('nz-slider', () => {
       expect(sliderInstance.value).toEqual([2, 99]);
     });
 
+    it('should trigger nzOnAfterChange', () => {
+      const onChangeSpy = jasmine.createSpy('slider onChange');
+
+      sliderInstance.nzOnAfterChange.subscribe(onChangeSpy);
+      dispatchKeyboardEvent(sliderNativeElement, 'keydown', RIGHT_ARROW);
+      fixture.detectChanges();
+
+      expect(onChangeSpy).toHaveBeenCalledTimes(1);
+    });
+
     it('should work for range slider when activeValueIndex is undefined', () => {
       testComponent.range = true;
       fixture.detectChanges();