fix(material-experimental/mdc-slider): collapse everything into slider.ts

* delete slider-adapter.ts
* delete slider-interface.ts
* delete slider-thumb.ts
* move SliderAdapter implementation to slider.ts
* move MatSliderThumb implementation to slider.ts
* go back to using direct references instead of interfaces
* fix imports in module.ts
* fix imports in public-api.ts

Author: wagnermaciel

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

@@ -9,8 +9,7 @@
 import {CommonModule} from '@angular/common';
 import {NgModule} from '@angular/core';
 import {MatCommonModule} from '@angular/material-experimental/mdc-core';
-import {MatSlider} from './slider';
-import {MatSliderThumb} from './slider-thumb';
+import {MatSlider, MatSliderThumb} from './slider';
 
 @NgModule({
   imports: [MatCommonModule, CommonModule],

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

@@ -6,6 +6,5 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-export {MatSlider} from './slider';
-export {MatSliderThumb} from './slider-thumb';
+export {MatSlider, MatSliderThumb, MatSliderDragEvent} from './slider';
 export {MatSliderModule} from './module';