fix(button-toggle): fix standalone button toggle style (#5121)

src/lib/button-toggle/button-toggle.scss

@@ -7,6 +7,7 @@ $mat-button-toggle-padding: 0 16px !default;
 $mat-button-toggle-height: 36px !default;
 $mat-button-toggle-border-radius: 2px !default;
 
+.mat-button-toggle-standalone,
 .mat-button-toggle-group {
   @include mat-elevation(2);
   position: relative;

src/lib/button-toggle/button-toggle.ts

@@ -268,6 +268,7 @@ export class MdButtonToggleGroupMultiple extends _MdButtonToggleGroupMixinBase
   styleUrls: ['button-toggle.css'],
   encapsulation: ViewEncapsulation.None,
   host: {
+    '[class.mat-button-toggle-standalone]': '!buttonToggleGroup && !buttonToggleGroupMultiple',
     'class': 'mat-button-toggle'
   }
 })