fix(material-experimental/mdc-slider): fix VE bug

* in view engine, MatSliders inputs are not initialized before MatSliderThumbs constructor is called.
  This means we cannot initialize the slider value attribute in the constructor. To fix this, we
  are initializing the value attribute in ngOnInit which is still before ngAfterViewInit but after
  MatSliders inputs are initialized

Author: wagnermaciel

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

@@ -28,6 +28,7 @@ import {
   Input,
   NgZone,
   OnDestroy,
+  OnInit,
   Output,
   QueryList,
   ViewChild,
@@ -260,7 +261,7 @@ export class MatSliderVisualThumb implements AfterViewInit, OnDestroy {
     multi: true
   }],
 })
-export class MatSliderThumb implements AfterViewInit, ControlValueAccessor {
+export class MatSliderThumb implements AfterViewInit, ControlValueAccessor, OnInit {
 
   // ** IMPORTANT NOTE **
   //
@@ -336,11 +337,14 @@ export class MatSliderThumb implements AfterViewInit, ControlValueAccessor {
     private readonly _elementRef: ElementRef<HTMLInputElement>) {
       this._document = document;
       this._hostElement = _elementRef.nativeElement;
-      // By calling this in the constructor we guarantee that the sibling sliders initial value by
-      // has already been set by the time we reach ngAfterViewInit().
-      this._initializeInputValueAttribute();
     }
 
+  ngOnInit() {
+    // By calling this in ngOnInit() we guarantee that the sibling sliders initial value by
+    // has already been set by the time we reach ngAfterViewInit().
+    this._initializeInputValueAttribute();
+  }
+
   ngAfterViewInit() {
     this._initializeInputState();
     this._initializeInputValueProperty();