fix(material-experimental/mdc-slider): code review changes

* try to resolve injection error that only throws on ci

Author: wagnermaciel

src/material-experimental/mdc-slider/slider.ts

@@ -24,6 +24,7 @@ import {
   ElementRef,
   EventEmitter,
   Inject,
+  Injectable,
   Input,
   NgZone,
   OnDestroy,
@@ -453,6 +454,7 @@ export class MatSliderThumb implements AfterViewInit, ControlValueAccessor {
 
 // Boilerplate for applying mixins to MatSlider.
 /** @docs-private */
+@Injectable()
 class MatSliderBase {
   constructor(public _elementRef: ElementRef<HTMLElement>) {}
 }